2653 lines
124 KiB
HTML
2653 lines
124 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<meta name="generator" content="Asciidoctor 1.5.8">
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<meta name="author" content="Peter Dimov">
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<title>Simple C++11 metaprogramming, part 2</title>
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</head>
|
|
<body class="article toc2 toc-left">
|
|
<div id="header">
|
|
<h1>Simple C++11 metaprogramming, part 2</h1>
|
|
<div class="details">
|
|
<span id="author" class="author">Peter Dimov</span><br>
|
|
<span id="revdate">2015-06-20</span>
|
|
</div>
|
|
<div id="toc" class="toc2">
|
|
<div id="toctitle">Table of Contents</div>
|
|
<ul class="sectlevel1">
|
|
<li><a href="#vectors_sets_and_maps">Vectors, sets, and maps</a></li>
|
|
<li><a href="#mp_contains">mp_contains</a></li>
|
|
<li><a href="#mp_map_find">mp_map_find</a></li>
|
|
<li><a href="#mp_at">mp_at</a></li>
|
|
<li><a href="#mp_drop">mp_drop</a></li>
|
|
<li><a href="#mp_find_index">mp_find_index</a></li>
|
|
<li><a href="#conclusion">Conclusion</a></li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div id="content">
|
|
<div id="preamble">
|
|
<div class="sectionbody">
|
|
<div class="paragraph lead">
|
|
<p><em>Efficient algorithms for membership testing, random access, and retrieval by
|
|
key</em></p>
|
|
</div>
|
|
<div class="admonitionblock note">
|
|
<table>
|
|
<tr>
|
|
<td class="icon">
|
|
<div class="title">Note</div>
|
|
</td>
|
|
<td class="content">
|
|
Being late to the metaprogramming party, I make no claim of having
|
|
invented the techniques in this article. A quick look at the implementations
|
|
of, for example, Louis Dionne’s <a href="https://github.com/ldionne/mpl11">mpl11</a> and
|
|
Eric Niebler’s <a href="https://github.com/ericniebler/meta">meta</a>, shows that most of
|
|
these tricks are already known. Dave Abrahams
|
|
<a href="https://github.com/dabrahams/mpl11">has experimented</a> along these lines in 2012.
|
|
The original inventor of the multiple inheritance trick and the <code>void*</code>
|
|
arguments trick is probably Richard Smith, who has posted
|
|
<a href="https://llvm.org/bugs/attachment.cgi?id=8825">two</a>
|
|
<a href="https://llvm.org/bugs/attachment.cgi?id=8838">examples</a> in response to
|
|
<a href="https://llvm.org/bugs/show_bug.cgi?id=13263">a Clang bug report</a>.
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="vectors_sets_and_maps">Vectors, sets, and maps</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p><a href="simple_cxx11_metaprogramming.html">Last time</a>, I outlined a style of
|
|
metaprogramming that operated on type lists — variadic class templates:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... T> struct mp_list {};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Classic Lisp uses lists as its only data structure, but operating on a list is
|
|
usually linear in the number of its elements.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>In addition to <code>list</code>, the STL has <code>vector</code>, <code>set</code>, and <code>map</code>. <code>vector</code>
|
|
supports random access by index; <code>set</code> has efficient test for membership; <code>map</code>
|
|
associates keys with values and has efficient lookup based on key.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Instead of introducing separate data structure such as <code>mp_vector</code>, <code>mp_set</code>,
|
|
<code>mp_map</code>, we’ll keep our data in a list form, and attempt to provide efficient
|
|
algorithms for random access, membership testing, and lookup.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="mp_contains">mp_contains</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p>Let’s starts with sets. A set is just a list with unique elements. To obtain a
|
|
set from an arbitrary list, we’ll need an algorithm that removes the
|
|
duplicates. Let’s call it <code>mp_unique<L></code>:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>// mp_if
|
|
|
|
template<bool C, class T, class E> struct mp_if_c_impl;
|
|
|
|
template<class T, class E> struct mp_if_c_impl<true, T, E>
|
|
{
|
|
using type = T;
|
|
};
|
|
|
|
template<class T, class E> struct mp_if_c_impl<false, T, E>
|
|
{
|
|
using type = E;
|
|
};
|
|
|
|
template<bool C, class T, class E>
|
|
using mp_if_c = typename mp_if_c_impl<C, T, E>::type;
|
|
|
|
template<class C, class T, class E>
|
|
using mp_if = typename mp_if_c_impl<C::value != 0, T, E>::type;
|
|
|
|
// mp_unique
|
|
|
|
template<class L> struct mp_unique_impl;
|
|
|
|
template<class L> using mp_unique = typename mp_unique_impl<L>::type;
|
|
|
|
template<template<class...> class L> struct mp_unique_impl<L<>>
|
|
{
|
|
using type = L<>;
|
|
};
|
|
|
|
template<template<class...> class L, class T1, class... T>
|
|
struct mp_unique_impl<L<T1, T...>>
|
|
{
|
|
using _rest = mp_unique<L<T...>>;
|
|
using type = mp_if<<strong>mp_contains</strong><_rest, T1>, _rest, mp_push_front<_rest, T1>>;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>For membership testing, we’ve introduced an algorithm <code>mp_contains<L, V></code> that
|
|
returns <code>true</code> when <code>L</code> contains <code>V</code>. The straightforward recursive
|
|
implementation of <code>mp_contains</code> is:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class L, class V> struct mp_contains_impl;
|
|
|
|
template<class L, class V> using mp_contains = typename mp_contains_impl<L, V>::type;
|
|
|
|
template<template<class...> class L, class V> struct mp_contains_impl<L<>, V>
|
|
{
|
|
using type = std::false_type;
|
|
};
|
|
|
|
template<template<class...> class L, class... T, class V>
|
|
struct mp_contains_impl<L<V, T...>, V>
|
|
{
|
|
using type = std::true_type;
|
|
};
|
|
|
|
template<template<class...> class L, class T1, class... T, class V>
|
|
struct mp_contains_impl<L<T1, T...>, V>: mp_contains_impl<L<T...>, V>
|
|
{
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Note that <code>mp_unique<L></code> makes <code>N</code> calls to <code>mp_contains</code>, where <code>N</code> is the
|
|
length of the list <code>L</code>. This means that <code>mp_contains</code> needs to be as fast as
|
|
possible, which the above implementation, well, isn’t.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Here are the compile times in seconds for invoking <code>mp_unique</code> on a list with
|
|
<code>N</code> (distinct) elements:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">13.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">30.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">10.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">23.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>(Tests done under Windows/Cygwin. All compilers are 32 bit. No optimizations.
|
|
DNF stands for "did not finish", which usually means that the compiler ran out
|
|
of heap space or crashed.)</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>We clearly need a better alternative.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>I ended the previous article with an implementation of <code>mp_contains</code> that
|
|
relied on <code>mp_count</code>, which in turn relied on <code>mp_plus</code>. Let’s see how it
|
|
fares:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, mp_count/mp_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">9.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">50.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, mp_count/mp_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">6.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, mp_count/mp_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">9.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">15.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">22.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">32.3</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>Not <em>that</em> bad, at least if your compiler happens to be <code>g++</code>. Still, there
|
|
ought to be room for improvement here.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>To do better, we have to somehow leverage the language features, such as pack
|
|
expansion, to do more of the work for us. For inspiration, let’s turn to
|
|
section 14.5.3 paragraph 4 of the C++11 standard, which explains that pack
|
|
expansions can occur in the following contexts:</p>
|
|
</div>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p><strong>In a function parameter pack (8.3.5); the pattern is the
|
|
<em>parameter-declaration</em> without the ellipsis.</strong></p>
|
|
</li>
|
|
<li>
|
|
<p>In a template parameter pack that is a pack expansion (14.1):</p>
|
|
</li>
|
|
<li>
|
|
<p><strong>In an <em>initializer-list</em> (8.5); the pattern is an
|
|
<em>initializer-clause</em>.</strong></p>
|
|
</li>
|
|
<li>
|
|
<p><strong>In a <em>base-specifier-list</em> (Clause 10); the pattern is a
|
|
<em>base-specifier</em>.</strong></p>
|
|
</li>
|
|
<li>
|
|
<p>In a <em>mem-initializer-list</em> (12.6.2); the pattern is a
|
|
<em>mem-initializer</em>.</p>
|
|
</li>
|
|
<li>
|
|
<p>In a <em>template-argument-list</em> (14.3); the pattern is a
|
|
<em>template-argument</em>.</p>
|
|
</li>
|
|
<li>
|
|
<p>In a <em>dynamic-exception-specification</em> (15.4); the pattern is a
|
|
<em>type-id</em>.</p>
|
|
</li>
|
|
<li>
|
|
<p>In an <em>attribute-list</em> (7.6.1); the pattern is an <em>attribute</em>.</p>
|
|
</li>
|
|
<li>
|
|
<p>In an <em>alignment-specifier</em> (7.6.2); the pattern is the
|
|
<em>alignment-specifier</em> without the ellipsis.</p>
|
|
</li>
|
|
<li>
|
|
<p>In a <em>capture-list</em> (5.1.2); the pattern is a <em>capture</em>.</p>
|
|
</li>
|
|
<li>
|
|
<p>In a <code>sizeof...</code> expression (5.3.3); the pattern is an <em>identifier</em>.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The <strong>emphasis</strong> is mine and indicates possible leads.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Our first option is to expand the parameter pack into arguments for a function
|
|
call. Since we’re interested in operations that occur at compile time, calling
|
|
a function may not appear useful; but C++11 functions can be <code>constexpr</code>, and
|
|
<code>constexpr</code> function "calls" do occur at compile time.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Recall our <code>mp_count</code>:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class L, class V> struct mp_count_impl;
|
|
|
|
template<template<class...> class L, class... T, class V>
|
|
struct mp_count_impl<L<T...>, V>
|
|
{
|
|
using type = mp_plus<std::is_same<T, V>...>;
|
|
};
|
|
|
|
template<class L, class V> using mp_count = typename mp_count_impl<L, V>::type;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Instead of using the template alias <code>mp_plus</code> to sum the <code>is_same</code> expressions,
|
|
we can use a <code>constexpr</code> function:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>constexpr std::size_t cx_plus()
|
|
{
|
|
return 0;
|
|
}
|
|
|
|
template<class T1, class... T> constexpr std::size_t cx_plus(T1 t1, T... t)
|
|
{
|
|
return t1 + cx_plus(t...);
|
|
}
|
|
|
|
// mp_size_t
|
|
|
|
template<std::size_t N> using mp_size_t = std::integral_constant<std::size_t, N>;
|
|
|
|
// mp_count
|
|
|
|
template<class L, class V> struct mp_count_impl;
|
|
|
|
template<template<class...> class L, class... T, class V>
|
|
struct mp_count_impl<L<T...>, V>
|
|
{
|
|
using type = mp_size_t<cx_plus(std::is_same<T, V>::value...)>;
|
|
};
|
|
|
|
template<class L, class V> using mp_count = typename mp_count_impl<L, V>::type;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>with the following results:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, mp_count/cx_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, mp_count/cx_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">6.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">9.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">11.8</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>We’ve improved the times, but lost VC++ 2013 due to its not implementing
|
|
<code>constexpr</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Let’s try pack expansion into an <em>initializer-list</em>. Instead of passing the
|
|
<code>is_same</code> expressions to a function, we can build a constant array out of them,
|
|
then sum the array with a <code>constexpr</code> function:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>constexpr std::size_t cx_plus2(bool const * first, bool const * last)
|
|
{
|
|
return first == last? 0: *first + cx_plus2(first + 1, last);
|
|
}
|
|
|
|
// mp_count
|
|
|
|
template<class L, class V> struct mp_count_impl;
|
|
|
|
template<template<class...> class L, class... T, class V>
|
|
struct mp_count_impl<L<T...>, V>
|
|
{
|
|
static constexpr bool _v[] = { std::is_same<T, V>::value... };
|
|
using type = mp_size_t<cx_plus2(_v, _v + sizeof...(T))>;
|
|
};
|
|
|
|
template<class L, class V> using mp_count = typename mp_count_impl<L, V>::type;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This is a neat trick, but is it fast?</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, mp_count/cx_plus2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, mp_count/cx_plus2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">7.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">11.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">14.7</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>That’s a bit disappointing. Let’s see what can we do with expanding a parameter
|
|
pack into a base-specifier-list. We would be able to define a class that
|
|
derives from every element of the pack:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>struct U: T... {};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>We can then use <code>std::is_base_of<V, U></code> to test whether a type <code>V</code> is a base of
|
|
<code>U</code>, that is, whether it’s one of the elements of the parameter pack. Which is
|
|
exactly what we need.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Arbitrary types such as <code>void</code>, <code>int</code>, or <code>void(int)</code> can’t be used as base
|
|
classes, but we’ll wrap the types in an empty class template, which we’ll call
|
|
<code>mp_identity</code>.</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class T> struct mp_identity
|
|
{
|
|
using type = T;
|
|
};
|
|
|
|
template<class L, class V> struct mp_contains_impl;
|
|
|
|
template<class L, class V> using mp_contains = typename mp_contains_impl<L, V>::type;
|
|
|
|
template<template<class...> class L, class... T, class V>
|
|
struct mp_contains_impl<L<T...>, V>
|
|
{
|
|
struct U: mp_identity<T>... {};
|
|
using type = std::is_base_of<mp_identity<V>, U>;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Performance?</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, is_base_of</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, is_base_of</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, is_base_of</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.0</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>This implementation is a clear winner.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>In fairness, we ought to note that the first four implementations of
|
|
<code>mp_contains</code> do not rely on the list elements being unique. This makes
|
|
<code>mp_contains</code> an algorithm that supports arbitrary lists, not just sets.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The <code>is_base_of</code> implementation, however, does not support lists that contain
|
|
duplicates, because it’s not possible to inherit directly from the same type
|
|
twice. So it does not implement the general <code>mp_contains</code>, but something that
|
|
should probably be named <code>mp_set_contains</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>We can avoid the "no duplicates" requirement by modifying the implementation to
|
|
inherit from <code>mp_identity<T></code> indirectly, via an intermediate base class:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>// indirect_inherit
|
|
|
|
template<std::size_t I, class T> struct inherit_second: T {};
|
|
|
|
template<class L, class S> struct indirect_inherit_impl;
|
|
|
|
template<template<class...> class L, class... T, std::size_t... J>
|
|
struct indirect_inherit_impl<L<T...>, <a href="http://en.cppreference.com/w/cpp/utility/integer_sequence">integer_sequence</a><std::size_t, J...>>:
|
|
inherit_second<J, mp_identity<T>>... {};
|
|
|
|
template<class L> using indirect_inherit =
|
|
indirect_inherit_impl<L, <a href="http://en.cppreference.com/w/cpp/utility/integer_sequence">make_index_sequence</a><mp_size<L>::value>>;
|
|
|
|
// mp_contains
|
|
|
|
template<class L, class V> struct mp_contains_impl
|
|
{
|
|
using U = indirect_inherit<L>;
|
|
using type = std::is_base_of<mp_identity<V>, U>;
|
|
};
|
|
|
|
template<class L, class V> using mp_contains = typename mp_contains_impl<L, V>::type;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This, however, pretty much nullifies the spectacular performance gains we’ve
|
|
observed with the original <code>is_base_of</code>-based implementation:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, mp_count/mp_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">9.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">50.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, is_base_of</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, is_base_of/indirect</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">9.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">49.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">153.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">13.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">30.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, mp_count/mp_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">6.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, mp_count/cx_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, mp_count/cx_plus2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, is_base_of</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, is_base_of/indirect</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">10.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">23.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, mp_count/mp_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">9.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">15.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">22.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">32.3</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, mp_count/cx_plus</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">6.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">9.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">11.8</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, mp_count/cx_plus2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">7.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">11.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">14.7</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, is_base_of</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.0</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, is_base_of/indirect</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">6.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">9.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">13.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">18.2</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="mp_map_find">mp_map_find</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p>A map, in the STL sense, is a data structure that associates keys with values
|
|
and can efficiently retrieve, given a key, its associated value. For our
|
|
purposes, a map will be any list of lists for which the inner lists have at
|
|
least one element, the key; the rest of the elements we’ll consider to be the
|
|
associated value. For example, the list</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>[[A, B], [C, D, E], [F], [G, H]]</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>is a map with keys <code>A</code>, <code>C</code>, <code>F</code>, and <code>G</code>, with associated values <code>[B]</code>,
|
|
<code>[D, E]</code>, <code>[]</code>, and <code>[H]</code>, respectively. We’ll require unique keys, for reasons
|
|
that’ll become evident later.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>I’ll show two other examples of maps, this time using real C++ code:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>using Map = mp_list<mp_list<int, int*>, mp_list<void, void*>, mp_list<char, char*>>;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>using Map2 = std::tuple<std::pair<int, int[2]>, std::pair<char, char[2]>>;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The Lisp name of the algorithm that performs retrieval based on key is <code>ASSOC</code>,
|
|
but I’ll call it <code>mp_map_find</code>. <code>mp_map_find<M, K></code> returns the element of <code>M</code>
|
|
whose first element is <code>K</code>. For example, <code>mp_map_find<Map2, int></code> would return
|
|
<code>std::pair<int, int[2]></code>. If there’s no such key, it returns <code>void</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>There’s almost no need to implement and benchmark the recursive version of
|
|
<code>mp_map_find</code> — we can be pretty sure it will perform horribly. Still,</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class M, class K> struct mp_map_find_impl;
|
|
|
|
template<class M, class K> using mp_map_find = typename mp_map_find_impl<M, K>::type;
|
|
|
|
template<template<class...> class M, class K> struct mp_map_find_impl<M<>, K>
|
|
{
|
|
using type = void;
|
|
};
|
|
|
|
template<template<class...> class M, class T1, class... T, class K>
|
|
struct mp_map_find_impl<M<T1, T...>, K>
|
|
{
|
|
using type = mp_if<std::is_same<mp_front<T1>, K>, T1, mp_map_find<M<T...>, K>>;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The compile time, in seconds, for <code>N</code> lookups into a map of size <code>N</code>, is as
|
|
follows:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">38.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">13.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">10.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">28.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>I told you there was no point.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>But, I hear some of you say, you’re evaluating the else branch even if the
|
|
condition is true, and that’s horribly inefficient!</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Well, this would only improve the performance by a factor of approximately two
|
|
on average, and only if the element is present, but fine, let’s try it. The
|
|
element happens to be present in the benchmark, so let’s see.</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>// mp_eval_if
|
|
|
|
template<bool C, class T, template<class...> class E, class... A>
|
|
struct mp_eval_if_c_impl;
|
|
|
|
template<class T, template<class...> class E, class... A>
|
|
struct mp_eval_if_c_impl<true, T, E, A...>
|
|
{
|
|
using type = T;
|
|
};
|
|
|
|
template<class T, template<class...> class E, class... A>
|
|
struct mp_eval_if_c_impl<false, T, E, A...>
|
|
{
|
|
using type = E<A...>;
|
|
};
|
|
|
|
template<bool C, class T, template<class...> class E, class... A>
|
|
using mp_eval_if_c = typename mp_eval_if_c_impl<C, T, E, A...>::type;
|
|
|
|
template<class C, class T, template<class...> class E, class... A>
|
|
using mp_eval_if = typename mp_eval_if_c_impl<C::value != 0, T, E, A...>::type;
|
|
|
|
// mp_map_find
|
|
|
|
template<class M, class K> struct mp_map_find_impl;
|
|
|
|
template<class M, class K> using mp_map_find = typename mp_map_find_impl<M, K>::type;
|
|
|
|
template<template<class...> class M, class K> struct mp_map_find_impl<M<>, K>
|
|
{
|
|
using type = void;
|
|
};
|
|
|
|
template<template<class...> class M, class T1, class... T, class K>
|
|
struct mp_map_find_impl<M<T1, T...>, K>
|
|
{
|
|
using type = mp_eval_if<std::is_same<mp_front<T1>, K>, T1, mp_map_find, M<T...>, K>;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>There you go:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">15.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">9.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">7.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">19.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>I told you there was no point.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Point or no, to establish that the recursive implementation is inefficient is
|
|
not the same as to come up with an efficient one. There are two things that
|
|
make the <code>mp_contains</code> techniques inapplicable to our present case: first,
|
|
<code>mp_contains</code> only had to return true or false, whereas <code>mp_map_find</code> returns a
|
|
type, and second, in <code>mp_contains</code> we knew the exact type of the element for
|
|
which we were looking, whereas here, we only know its <code>mp_front</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Fortunately, there does exist a language feature that can solve both: C++ can
|
|
deduce the template parameters of base classes when passed a derived class. In
|
|
this example,</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>struct K1 {};
|
|
struct V1 {};
|
|
|
|
struct X: std::pair<K1, V1> {};
|
|
|
|
template<class A, class B> void f(std::pair<A, B> const & p);
|
|
|
|
int main()
|
|
{
|
|
f(X());
|
|
}</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>the call to <code>f(X())</code> deduces <code>A</code> as <code>K1</code> and <code>B</code> as <code>V1</code>. If we have more than
|
|
one <code>std::pair</code> base class, we can fix <code>A</code> to be <code>K1</code>:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>struct K1 {};
|
|
struct V1 {};
|
|
|
|
struct K2 {};
|
|
struct V2 {};
|
|
|
|
struct X: std::pair<K1, V1>, std::pair<K2, V2> {};
|
|
|
|
template<class B> void f(std::pair<K1, B> const & p);
|
|
|
|
int main()
|
|
{
|
|
f(X());
|
|
}</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>and <code>B</code> will be deduced as <code>V1</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>We can retrieve the results of the deduction by returning the type we want:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class B> std::pair<K1, B> f(std::pair<K1, B> const & p);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>and then using <code>decltype(f(X()))</code> to obtain this return type.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>What if <code>X</code> doesn’t have a base of type <code>std::pair<K1, B></code>? The deduction will
|
|
fail and we’ll get an error that <code>f(X())</code> cannot be called. To avoid it, we can
|
|
add an overload of <code>f</code> that takes anything and returns <code>void</code>. But in this
|
|
case, what will happen if <code>X</code> has two bases of the form that match the first
|
|
<code>f</code> overload, such as for example <code>std::pair<K1, Y></code> and <code>std::pair<K1, Z></code>?</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The deduction will fail, the second overload will again be chosen and we’ll get
|
|
<code>void</code>. This is why we require maps to have unique keys.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Here’s an implementation of <code>mp_map_find</code> based on this technique:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class M, class K> struct mp_map_find_impl;
|
|
|
|
template<class M, class K>
|
|
using mp_map_find = typename mp_map_find_impl<M, K>::type;
|
|
|
|
template<template<class...> class M, class... T, class K>
|
|
struct mp_map_find_impl<M<T...>, K>
|
|
{
|
|
struct U: mp_identity<T>... {};
|
|
|
|
template<template<class...> class L, class... U>
|
|
static mp_identity<L<K, U...>>
|
|
f( mp_identity<L<K, U...>>* );
|
|
|
|
static mp_identity<void> f( ... );
|
|
|
|
using V = decltype( f((U*)0) );
|
|
|
|
using type = typename V::type;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>and its corresponding compile times:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, deduction</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">6.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">10.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">16.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, deduction</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.7</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, deduction</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">6.3</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>This looks ready to ship.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The implementation contains one inefficiency though. If we evaluate
|
|
<code>mp_map_find<M, K1></code>, then <code>mp_map_find<M, K2></code>, the two nested <code>U</code> types are
|
|
the same as they only depend on <code>M</code>, but the compiler doesn’t know that and
|
|
will instantiate each one separately. We should move this type outside
|
|
<code>mp_map_find_impl</code> so that it can be reused:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... T> struct <strong>mp_inherit</strong>: T... {};
|
|
|
|
template<class M, class K> struct mp_map_find_impl;
|
|
|
|
template<class M, class K>
|
|
using mp_map_find = typename mp_map_find_impl<M, K>::type;
|
|
|
|
template<template<class...> class M, class... T, class K>
|
|
struct mp_map_find_impl<M<T...>, K>
|
|
{
|
|
<strong>using U = mp_inherit<mp_identity<T>...>;</strong>
|
|
|
|
template<template<class...> class L, class... U>
|
|
static mp_identity<L<K, U...>>
|
|
f( mp_identity<L<K, U...>>* );
|
|
|
|
static mp_identity<void> f( ... );
|
|
|
|
using V = decltype( f((U*)0) );
|
|
|
|
using type = typename V::type;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>(This same optimization, by the way, applies to our <code>is_base_of</code> implementation
|
|
of <code>mp_contains</code>.)</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The improvement in compile times on our benchmark is measurable:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, deduction+mp_inherit</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">7.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">10.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, deduction+mp_inherit</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.2</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, deduction+mp_inherit</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.9</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="mp_at">mp_at</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p>With sets and maps covered, it’s time to tackle vectors. Vectors for us are
|
|
just lists, to which we’ll need to add the ability to efficiently access an
|
|
element based on its index. The customary name for this accessor is
|
|
<code>mp_at<L, I></code>, where <code>L</code> is a list and <code>I</code> is an <code>integral_constant</code> that
|
|
represents the index. We’ll also follow the Boost.MPL convention and add
|
|
<code>mp_at_c<L, I></code>, where <code>I</code> is the index of type <code>size_t</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The recursive implementation of <code>mp_at</code> is:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class L, std::size_t I> struct mp_at_c_impl;
|
|
|
|
template<class L, std::size_t I> using mp_at_c = typename mp_at_c_impl<L, I>::type;
|
|
|
|
template<class L, class I> using mp_at = typename mp_at_c_impl<L, I::value>::type;
|
|
|
|
template<template<class...> class L, class T1, class... T>
|
|
struct mp_at_c_impl<L<T1, T...>, 0>
|
|
{
|
|
using type = T1;
|
|
};
|
|
|
|
template<template<class...> class L, class T1, class... T, std::size_t I>
|
|
struct mp_at_c_impl<L<T1, T...>, I>
|
|
{
|
|
using type = mp_at_c<L<T...>, I-1>;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>and the compile times for making <code>N</code> calls to <code>mp_at</code> with a list of size <code>N</code>
|
|
as the first argument are:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">15.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">14.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">32.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>To improve upon this appalling result, we’ll again exploit pack expansion into a
|
|
function call, but in a novel way. Let’s suppose that we need to access the
|
|
fourth element (<code>I = 3</code>). We’ll generate the function signature</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class W> W f( void*, void*, void*, W*, ... );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>and then, given a list <code>L<T1, T2, T3, T4, T5, T6, T7></code>, we’ll evaluate the
|
|
expression</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>decltype( f( (T1*)0, (T2*)0, (T3*)0, (T4*)0, (T5*)0, (T6*)0, (T7*)0 ) )</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The three <code>void*</code> parameters will eat the first three elements, <code>W</code> will be
|
|
deduced as the fourth, and the ellipsis will take care of the rest.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>A working implementation based on this technique is shown below:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>// mp_repeat_c
|
|
|
|
template<std::size_t N, class... T> struct mp_repeat_c_impl
|
|
{
|
|
using _l1 = typename mp_repeat_c_impl<N/2, T...>::type;
|
|
using _l2 = typename mp_repeat_c_impl<N%2, T...>::type;
|
|
|
|
using type = mp_append<_l1, _l1, _l2>;
|
|
};
|
|
|
|
template<class... T> struct mp_repeat_c_impl<0, T...>
|
|
{
|
|
using type = mp_list<>;
|
|
};
|
|
|
|
template<class... T> struct mp_repeat_c_impl<1, T...>
|
|
{
|
|
using type = mp_list<T...>;
|
|
};
|
|
|
|
template<std::size_t N, class... T> using mp_repeat_c =
|
|
typename mp_repeat_c_impl<N, T...>::type;
|
|
|
|
// mp_at
|
|
|
|
template<class L, class L2> struct mp_at_c_impl;
|
|
|
|
template<template<class...> class L, class... T,
|
|
template<class...> class L2, class... U>
|
|
struct mp_at_c_impl<L<T...>, L2<U...>>
|
|
{
|
|
template<class W> static W f( U*..., W*, ... );
|
|
|
|
using R = decltype( f( (mp_identity<T>*)0 ... ) );
|
|
|
|
using type = typename R::type;
|
|
};
|
|
|
|
template<class L, std::size_t I> using mp_at_c =
|
|
typename mp_at_c_impl<L, mp_repeat_c<I, void>>::type;
|
|
|
|
template<class L, class I> using mp_at = mp_at_c<L, I::value>;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>and the compile times in the following table show it to be good enough for most
|
|
practical purposes.</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, void*</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, void*</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">6.6</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, void*</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.5</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>Are we done with <code>mp_at</code>, then?</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Let’s try something else — transform the input list <code>[T1, T2, T3]</code> into a map
|
|
<code>[[0, T1], [1, T2], [2, T3]]</code>, then use <code>mp_map_find</code> for the lookup:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>// mp_map_from_list
|
|
|
|
template<class L, class S> struct mp_map_from_list_impl;
|
|
|
|
template<template<class...> class L, class... T, std::size_t... J>
|
|
struct mp_map_from_list_impl<L<T...>, <a href="http://en.cppreference.com/w/cpp/utility/integer_sequence">integer_sequence</a><std::size_t, J...>>
|
|
{
|
|
using type = mp_list<mp_list<mp_size_t<J>, T>...>;
|
|
};
|
|
|
|
template<class L> using mp_map_from_list = typename mp_map_from_list_impl<L,
|
|
<a href="http://en.cppreference.com/w/cpp/utility/integer_sequence">make_index_sequence</a><mp_size<L>::value>>::type;
|
|
|
|
// mp_at
|
|
|
|
template<class L, std::size_t I> struct mp_at_c_impl
|
|
{
|
|
using map = mp_map_from_list<L>;
|
|
using type = mp_second<mp_map_find<map, mp_size_t<I>>>;
|
|
};
|
|
|
|
template<class L, std::size_t I> using mp_at_c = typename mp_at_c_impl<L, I>::type;
|
|
|
|
template<class L, class I> using mp_at = typename mp_at_c_impl<L, I::value>::type;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>At first sight, this looks ridiculous, but metaprogramming has its own rules.
|
|
Let’s measure:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, map</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">7.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">11.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, map</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.3</p></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, map</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.2</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>Surprise, this is the best implementation.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="mp_drop">mp_drop</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p>It turned out that we didn’t need the <code>void*</code> trick for <code>mp_at</code>, but I’ll show
|
|
an example where we do: <code>mp_drop</code>. <code>mp_drop<L, N></code> returns the list <code>L</code> without
|
|
its first <code>N</code> elements; or, in other words, it drops the first <code>N</code> elements
|
|
(presumably on the cutting room floor) and returns what’s left.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>To implement <code>mp_drop</code>, we just need to change</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class W> W f( void*, void*, void*, W*, ... );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>from above to return the rest of the elements, rather than just one:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... W> mp_list<W> f( void*, void*, void*, W*... );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Adding the necessary <code>mp_identity</code> seasoning produces the following working
|
|
implementation:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class L, class L2> struct mp_drop_c_impl;
|
|
|
|
template<template<class...> class L, class... T,
|
|
template<class...> class L2, class... U>
|
|
struct mp_drop_c_impl<L<T...>, L2<U...>>
|
|
{
|
|
template<class... W> static mp_identity<L<W...>> f( U*..., mp_identity<W>*... );
|
|
|
|
using R = decltype( f( (mp_identity<T>*)0 ... ) );
|
|
|
|
using type = typename R::type;
|
|
};
|
|
|
|
template<class L, std::size_t N> using mp_drop_c =
|
|
typename mp_drop_c_impl<L, mp_repeat_c<N, void>>::type;
|
|
|
|
template<class L, class N> using mp_drop = mp_drop_c<L, N::value>;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>I’ll skip the recursive implementation and the performance comparison for this
|
|
one. We can pretty much tell who’s going to win, and by how much.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="mp_find_index">mp_find_index</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p>The final algorithm that I’ll bring to your attention is <code>mp_find_index</code>.
|
|
<code>mp_find_index<L, V></code> returns an integral constant of type <code>size_t</code> with a
|
|
value that is the index of the first occurrence of <code>V</code> in <code>L</code>. If <code>V</code> is not in
|
|
<code>L</code>, the return value is the size of <code>L</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>We’ll start with the recursive implementation, as usual:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class L, class V> struct mp_find_index_impl;
|
|
|
|
template<class L, class V> using mp_find_index = typename mp_find_index_impl<L, V>::type;
|
|
|
|
template<template<class...> class L, class V> struct mp_find_index_impl<L<>, V>
|
|
{
|
|
using type = mp_size_t<0>;
|
|
};
|
|
|
|
template<template<class...> class L, class... T, class V>
|
|
struct mp_find_index_impl<L<V, T...>, V>
|
|
{
|
|
using type = mp_size_t<0>;
|
|
};
|
|
|
|
template<template<class...> class L, class T1, class... T, class V>
|
|
struct mp_find_index_impl<L<T1, T...>, V>
|
|
{
|
|
using type = mp_size_t<1 + mp_find_index<L<T...>, V>::value>;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>and will continue with the compile times for <code>N</code> calls to <code>mp_find_index</code> on a
|
|
list with <code>N</code> elements, as usual:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, recursive</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">13.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>What can we do here?</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Let’s go back to <code>mp_contains</code> and look at the "mp_count/cx_plus2"
|
|
implementation which we rejected in favor of the inheritance-based one. It
|
|
built a <code>constexpr</code> array of booleans and summed them in a <code>constexpr</code>
|
|
function. We can do the same here, except instead of summing the array, we can
|
|
find the index of the first true value:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class L, class V> struct mp_find_index_impl;
|
|
|
|
template<class L, class V> using mp_find_index = typename mp_find_index_impl<L, V>::type;
|
|
|
|
template<template<class...> class L, class V> struct mp_find_index_impl<L<>, V>
|
|
{
|
|
using type = mp_size_t<0>;
|
|
};
|
|
|
|
constexpr std::size_t cx_find_index( bool const * first, bool const * last )
|
|
{
|
|
return first == last || *first? 0: 1 + cx_find_index( first + 1, last );
|
|
}
|
|
|
|
template<template<class...> class L, class... T, class V>
|
|
struct mp_find_index_impl<L<T...>, V>
|
|
{
|
|
static constexpr bool _v[] = { std::is_same<T, V>::value... };
|
|
|
|
using type = mp_size_t< cx_find_index( _v, _v + sizeof...(T) ) >;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The performance of this version is:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, constexpr</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.9</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, constexpr</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">1.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">3.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">8.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">13.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">18.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>which, while not ideal, is significantly better than our recursive punching
|
|
bag. But if our compiler of choice is VC++ 2013, we can’t use <code>constexpr</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>We may attempt an implementation along the same lines, but with the <code>constexpr</code>
|
|
function replaced with ordinary metaprogramming:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class L, class V> struct mp_find_index_impl;
|
|
|
|
template<class L, class V> using mp_find_index = typename mp_find_index_impl<L, V>::type;
|
|
|
|
template<template<class...> class L, class V> struct mp_find_index_impl<L<>, V>
|
|
{
|
|
using type = mp_size_t<0>;
|
|
};
|
|
|
|
template<bool...> struct find_index_impl_;
|
|
|
|
template<> struct find_index_impl_<>
|
|
{
|
|
static const std::size_t value = 0;
|
|
};
|
|
|
|
template<bool B1, bool... R> struct find_index_impl_<B1, R...>
|
|
{
|
|
static const std::size_t value = B1? 0: 1 + find_index_impl_<R...>::value;
|
|
};
|
|
|
|
template<bool B1, bool B2, bool B3, bool B4, bool B5,
|
|
bool B6, bool B7, bool B8, bool B9, bool B10, bool... R>
|
|
struct find_index_impl_<B1, B2, B3, B4, B5, B6, B7, B8, B9, B10, R...>
|
|
{
|
|
static const std::size_t value = B1? 0: B2? 1: B3? 2: B4? 3: B5? 4:
|
|
B6? 5: B7? 6: B8? 7: B9? 8: B10? 9: 10 + find_index_impl_<R...>::value;
|
|
};
|
|
|
|
template<template<class...> class L, class... T, class V>
|
|
struct mp_find_index_impl<L<T...>, V>
|
|
{
|
|
using type = mp_size_t<find_index_impl_<std::is_same<T, V>::value...>::value>;
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This is still recursive, so we don’t expect miracles, but it wouldn’t hurt to
|
|
measure:</p>
|
|
</div>
|
|
<table class="tableblock frame-all grid-all stretch">
|
|
<colgroup>
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1111%;">
|
|
<col style="width: 11.1112%;">
|
|
</colgroup>
|
|
<thead>
|
|
<tr>
|
|
<th class="tableblock halign-left valign-top"></th>
|
|
<th class="tableblock halign-left valign-top">N=100</th>
|
|
<th class="tableblock halign-left valign-top">N=200</th>
|
|
<th class="tableblock halign-left valign-top">N=300</th>
|
|
<th class="tableblock halign-left valign-top">N=400</th>
|
|
<th class="tableblock halign-left valign-top">N=500</th>
|
|
<th class="tableblock halign-left valign-top">N=600</th>
|
|
<th class="tableblock halign-left valign-top">N=700</th>
|
|
<th class="tableblock halign-left valign-top">N=800</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">VC++ 2013, bool…​</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">4.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">94.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">488.3</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">XFA</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">clang++ 3.5.1, bool…​</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">5.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">12.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">21.7</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">35.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
<td class="tableblock halign-left valign-top"></td>
|
|
</tr>
|
|
<tr>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">g++ 4.9.2, bool…​</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">0.6</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">2.4</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">6.5</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">13.2</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">23.8</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">39.1</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">59.0</p></td>
|
|
<td class="tableblock halign-left valign-top"><p class="tableblock">DNF</p></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="paragraph">
|
|
<p>(where XFA stands for "experimenter fell asleep".)</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This is an interesting tradeoff for VC++ 2013 and Clang. On the one hand,
|
|
this implementation is slower; on the other, it doesn’t crash the compiler as
|
|
easily. Which to prefer is a matter of taste and of stern evaluation of one’s
|
|
needs to manipulate type lists of length 300.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Note that once we have <code>mp_drop</code> and <code>mp_find_index</code>, we can derive the
|
|
<code>mp_find<L, V></code> algorithm, which returns the suffix of <code>L</code> starting with the
|
|
first occurrence of <code>V</code>, if any, and an empty list otherwise, by using
|
|
<code>mp_drop<L, mp_find_index<L, V>></code>.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="conclusion">Conclusion</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p>In this article, I have shown efficient algorithms that allow us to treat type
|
|
lists as sets, maps and vectors, demonstrating various C++11 implementation
|
|
techniques in the process.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div id="footer">
|
|
<div id="footer-text">
|
|
Last updated 2021-04-13 16:22:27 UTC
|
|
</div>
|
|
</div>
|
|
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|
|
*:not(pre)>code { background: none; color: #600000; }
|
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