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<div><h2 class="title">
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<a name="boost_functional_forward"></a>Chapter 1. Boost.Functional/Forward 1.0</h2></div>
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<div><div class="author"><h3 class="author">
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<span class="firstname">Tobias</span> <span class="surname">Schwinger</span>
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</h3></div></div>
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<div><p class="copyright">Copyright © 2007, 2008 Tobias Schwinger</p></div>
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<div><div class="legalnotice">
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<a name="boost_functional_forward.legal"></a><p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl class="toc">
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<dt><span class="section"><a href="index.html#boost_functional_forward.brief_description">Brief Description</a></span></dt>
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<dt><span class="section"><a href="index.html#boost_functional_forward.background">Background</a></span></dt>
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<dt><span class="section"><a href="index.html#boost_functional_forward.reference">Reference</a></span></dt>
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<dt><span class="section"><a href="index.html#boost_functional_forward.acknowledgements">Acknowledgements</a></span></dt>
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<dt><span class="section"><a href="index.html#boost_functional_forward.references">References</a></span></dt>
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</dl>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_functional_forward.brief_description"></a><a class="link" href="index.html#boost_functional_forward.brief_description" title="Brief Description">Brief Description</a>
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</h2></div></div></div>
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<p>
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<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward_adapter</span></code> provides a reusable adapter
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template for function objects. It forwards RValues as references to const,
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while leaving LValues as-is.
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">g</span> <span class="comment">// function object that only accept LValues</span>
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<span class="special">{</span>
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<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T2</span> <span class="special">></span>
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<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T0</span> <span class="special">&</span> <span class="identifier">t0</span><span class="special">,</span> <span class="identifier">T1</span> <span class="special">&</span> <span class="identifier">t1</span><span class="special">,</span> <span class="identifier">T2</span> <span class="special">&</span> <span class="identifier">t2</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
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<span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
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<span class="special">};</span>
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<span class="comment">// Adapted version also accepts RValues and forwards</span>
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<span class="comment">// them as references to const, LValues as-is</span>
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<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward_adapter</span><span class="special"><</span><span class="identifier">g</span><span class="special">></span> <span class="identifier">f</span><span class="special">;</span>
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</pre>
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<p>
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Another adapter, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lighweight_forward_adapter</span></code> allows forwarding
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with some help from the user accepting and unwrapping reference wrappers (see
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<a href="http://www.boost.org/doc/html/ref.html" target="_top">Boost.Ref</a>) for
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reference arguments, const qualifying all other arguments.
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</p>
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<p>
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The target functions must be compatible with <a href="http://www.boost.org/libs/utility/utility.htm#result_of" target="_top">Boost.ResultOf</a>,
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and so are the adapters.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_functional_forward.background"></a><a class="link" href="index.html#boost_functional_forward.background" title="Background">Background</a>
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</h2></div></div></div>
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<p>
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Let's suppose we have some function <code class="computeroutput"><span class="identifier">f</span></code>
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that we can call like this:
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</p>
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<pre class="programlisting"><span class="identifier">f</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="identifier">a_variable</span><span class="special">);</span>
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</pre>
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<p>
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Now we want to write another, generic function <code class="computeroutput"><span class="identifier">g</span></code>
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that can be called the same way and returns some object that calls <code class="computeroutput"><span class="identifier">f</span></code> with the same arguments.
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</p>
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<pre class="programlisting"><span class="identifier">f</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="identifier">a_variable</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">g</span><span class="special">(</span><span class="identifier">f</span><span class="special">,</span><span class="number">123</span><span class="special">,</span><span class="identifier">a_variable</span><span class="special">).</span><span class="identifier">call_f</span><span class="special">()</span>
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</pre>
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<h4>
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<a name="boost_functional_forward.background.h0"></a>
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<span class="phrase"><a name="boost_functional_forward.background.why_would_we_want_to_do_it__anyway_"></a></span><a class="link" href="index.html#boost_functional_forward.background.why_would_we_want_to_do_it__anyway_">Why
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would we want to do it, anyway?</a>
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</h4>
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<p>
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Maybe we want to run <code class="computeroutput"><span class="identifier">f</span></code> several
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times. Or maybe we want to run it within another thread. Maybe we just want
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to encapsulate the call expression for now, and then use it with other code
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that allows to compose more complex expressions in order to decompose it with
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C++ templates and have the compiler generate some machinery that eventually
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calls <code class="computeroutput"><span class="identifier">f</span></code> at runtime (in other
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words; apply a technique that is commonly referred to as Expression Templates).
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</p>
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<h4>
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<a name="boost_functional_forward.background.h1"></a>
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<span class="phrase"><a name="boost_functional_forward.background.now__how_do_we_do_it_"></a></span><a class="link" href="index.html#boost_functional_forward.background.now__how_do_we_do_it_">Now, how
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do we do it?</a>
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</h4>
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<p>
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The bad news is: It's impossible.
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</p>
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<p>
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That is so because there is a slight difference between a variable and an expression
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that evaluates to its value: Given
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</p>
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<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">y</span><span class="special">;</span>
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<span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">z</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
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</pre>
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<p>
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and
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">></span> <span class="keyword">void</span> <span class="identifier">func1</span><span class="special">(</span><span class="identifier">T</span> <span class="special">&</span> <span class="identifier">x</span><span class="special">);</span>
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</pre>
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<p>
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we can call
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</p>
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<pre class="programlisting"><span class="identifier">func1</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span> <span class="comment">// x is a reference to a non-const object</span>
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<span class="identifier">func1</span><span class="special">(</span><span class="identifier">z</span><span class="special">);</span> <span class="comment">// x is a reference to a const object</span>
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</pre>
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<p>
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where
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</p>
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<pre class="programlisting"><span class="identifier">func1</span><span class="special">(</span><span class="number">1</span><span class="special">);</span> <span class="comment">// fails to compile.</span>
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</pre>
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<p>
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This way we can safely have <code class="computeroutput"><span class="identifier">func1</span></code>
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store its reference argument and the compiler keeps us from storing a reference
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to an object with temporary lifetime.
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</p>
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<p>
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It is important to realize that non-constness and whether an object binds to
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a non-const reference parameter are two different properties. The latter is
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the distinction between LValues and RValues. The names stem from the left hand
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side and the right hand side of assignment expressions, thus LValues are typically
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the ones you can assign to, and RValues the temporary results from the right
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hand side expression.
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</p>
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<pre class="programlisting"><span class="identifier">y</span> <span class="special">=</span> <span class="number">1</span><span class="special">+</span><span class="number">2</span><span class="special">;</span> <span class="comment">// a is LValue, 1+2 is the expression producing the RValue,</span>
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<span class="comment">// 1+2 = a; // usually makes no sense. </span>
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<span class="identifier">func1</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span> <span class="comment">// works, because y is an LValue</span>
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<span class="comment">// func1(1+2); // fails to compile, because we only got an RValue.</span>
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</pre>
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<p>
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If we add const qualification on the parameter, our function also accepts RValues:
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">></span> <span class="keyword">void</span> <span class="identifier">func2</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">x</span><span class="special">);</span>
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<span class="comment">// [...] function scope:</span>
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<span class="identifier">func2</span><span class="special">(</span><span class="number">1</span><span class="special">);</span> <span class="comment">// x is a reference to a const temporary, object,</span>
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<span class="identifier">func2</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span> <span class="comment">// x is a reference to a const object, while y is not const, and</span>
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<span class="identifier">func2</span><span class="special">(</span><span class="identifier">z</span><span class="special">);</span> <span class="comment">// x is a reference to a const object, just like z.</span>
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</pre>
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<p>
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In all cases, the argument <code class="computeroutput"><span class="identifier">x</span></code>
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in <code class="computeroutput"><span class="identifier">func2</span></code> is a const-qualified
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LValue. We can use function overloading to identify non-const LValues:
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">></span> <span class="keyword">void</span> <span class="identifier">func3</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">x</span><span class="special">);</span> <span class="comment">// #1</span>
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<span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">></span> <span class="keyword">void</span> <span class="identifier">func3</span><span class="special">(</span><span class="identifier">T</span> <span class="special">&</span> <span class="identifier">x</span><span class="special">);</span> <span class="comment">// #2</span>
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<span class="comment">// [...] function scope:</span>
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<span class="identifier">func3</span><span class="special">(</span><span class="number">1</span><span class="special">);</span> <span class="comment">// x is a reference to a const, temporary object in #1,</span>
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<span class="identifier">func3</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span> <span class="comment">// x is a reference to a non-const object in #2, and</span>
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<span class="identifier">func3</span><span class="special">(</span><span class="identifier">z</span><span class="special">);</span> <span class="comment">// x is a reference to a const object in #1.</span>
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</pre>
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<p>
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Note that all arguments <code class="computeroutput"><span class="identifier">x</span></code> in
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the overloaded function <code class="computeroutput"><span class="identifier">func3</span></code>
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are LValues. In fact, there is no way to transport RValues into a function
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as-is in C++98. Also note that we can't distinguish between what used to be
|
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a const qualified LValue and an RValue.
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</p>
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<p>
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That's as close as we can get to a generic forwarding function <code class="computeroutput"><span class="identifier">g</span></code> as described above by the means of C++
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98. See <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm" target="_top">The
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Forwarding Problem</a> for a very detailed discussion including solutions
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that require language changes.
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</p>
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<p>
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Now, for actually implementing it, we need 2^N overloads for N parameters (each
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with and without const qualifier) for each number of arguments (that is 2^(Nmax+1)
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- 2^Nmin). Right, that means the compile-time complexity is O(2^N), however
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the factor is low so it works quite well for a reasonable number (< 10)
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of arguments.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_functional_forward.reference"></a><a class="link" href="index.html#boost_functional_forward.reference" title="Reference">Reference</a>
|
||
</h2></div></div></div>
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<div class="toc"><dl class="toc">
|
||
<dt><span class="section"><a href="index.html#boost_functional_forward.reference.forward_adapter">forward_adapter</a></span></dt>
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<dt><span class="section"><a href="index.html#boost_functional_forward.reference.lightweight_forward_adapter">lightweight_forward_adapter</a></span></dt>
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||
</dl></div>
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||
<div class="section">
|
||
<div class="titlepage"><div><div><h3 class="title">
|
||
<a name="boost_functional_forward.reference.forward_adapter"></a><a class="link" href="index.html#boost_functional_forward.reference.forward_adapter" title="forward_adapter">forward_adapter</a>
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</h3></div></div></div>
|
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<h5>
|
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<a name="boost_functional_forward.reference.forward_adapter.h0"></a>
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<span class="phrase"><a name="boost_functional_forward.reference.forward_adapter.description"></a></span><a class="link" href="index.html#boost_functional_forward.reference.forward_adapter.description">Description</a>
|
||
</h5>
|
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<p>
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Function object adapter template whose instances are callable with LValue
|
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and RValue arguments. RValue arguments are forwarded as reference-to-const
|
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typed LValues.
|
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</p>
|
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<p>
|
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An arity can be given as second, numeric non-type template argument to restrict
|
||
forwarding to a specific arity. If a third, numeric non-type template argument
|
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is present, the second and third template argument are treated as minimum
|
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and maximum arity, respectively. Specifying an arity can be helpful to improve
|
||
the readability of diagnostic messages and compile time performance.
|
||
</p>
|
||
<p>
|
||
<a href="http://www.boost.org/libs/utility/utility.htm#result_of" target="_top">Boost.ResultOf</a>
|
||
can be used to determine the result types of specific call expressions.
|
||
</p>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.forward_adapter.h1"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.forward_adapter.header"></a></span><a class="link" href="index.html#boost_functional_forward.reference.forward_adapter.header">Header</a>
|
||
</h5>
|
||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">functional</span><span class="special">/</span><span class="identifier">forward_adapter</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||
</pre>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.forward_adapter.h2"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.forward_adapter.synopsis"></a></span><a class="link" href="index.html#boost_functional_forward.reference.forward_adapter.synopsis">Synopsis</a>
|
||
</h5>
|
||
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span>
|
||
<span class="special">{</span>
|
||
<span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Function</span><span class="special">,</span>
|
||
<span class="keyword">int</span> <span class="identifier">Arity_Or_MinArity</span> <span class="special">=</span> <span class="emphasis"><em>unspecified</em></span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">MaxArity</span> <span class="special">=</span> <span class="emphasis"><em>unspecified</em></span> <span class="special">></span>
|
||
<span class="keyword">class</span> <span class="identifier">forward_adapter</span><span class="special">;</span>
|
||
<span class="special">}</span>
|
||
</pre>
|
||
<div class="variablelist">
|
||
<p class="title"><b>Notation</b></p>
|
||
<dl class="variablelist">
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">F</span></code></span></dt>
|
||
<dd><p>
|
||
a possibly const qualified function object type or reference type thereof
|
||
</p></dd>
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">f</span></code></span></dt>
|
||
<dd><p>
|
||
an object convertible to <code class="computeroutput"><span class="identifier">F</span></code>
|
||
</p></dd>
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">FA</span></code></span></dt>
|
||
<dd><p>
|
||
the type <code class="computeroutput"><span class="identifier">forward_adapter</span><span class="special"><</span><span class="identifier">F</span><span class="special">></span></code>
|
||
</p></dd>
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">fa</span></code></span></dt>
|
||
<dd><p>
|
||
an instance object of <code class="computeroutput"><span class="identifier">FA</span></code>,
|
||
initialized with <code class="computeroutput"><span class="identifier">f</span></code>
|
||
</p></dd>
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">a0</span></code>...<code class="computeroutput"><span class="identifier">aN</span></code></span></dt>
|
||
<dd><p>
|
||
arguments to <code class="computeroutput"><span class="identifier">fa</span></code>
|
||
</p></dd>
|
||
</dl>
|
||
</div>
|
||
<p>
|
||
The result type of a target function invocation must be
|
||
</p>
|
||
<pre class="programlisting"><a href="http://www.boost.org/libs/utility/utility.htm#result_of" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span></code></a><span class="special"><</span><span class="identifier">F</span><span class="special">*(</span><span class="identifier">TA0</span> <span class="special">[</span><span class="keyword">const</span><span class="special">]&...</span><span class="identifier">TAN</span> <span class="special">[</span><span class="keyword">const</span><span class="special">]&])>::</span><span class="identifier">type</span>
|
||
</pre>
|
||
<p>
|
||
where <code class="computeroutput"><span class="identifier">TA0</span></code>...<code class="computeroutput"><span class="identifier">TAN</span></code> denote the argument types of <code class="computeroutput"><span class="identifier">a0</span></code>...<code class="computeroutput"><span class="identifier">aN</span></code>.
|
||
</p>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.forward_adapter.h3"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.forward_adapter.expression_semantics"></a></span><a class="link" href="index.html#boost_functional_forward.reference.forward_adapter.expression_semantics">Expression
|
||
Semantics</a>
|
||
</h5>
|
||
<div class="informaltable"><table class="table">
|
||
<colgroup>
|
||
<col>
|
||
<col>
|
||
</colgroup>
|
||
<thead><tr>
|
||
<th>
|
||
<p>
|
||
Expression
|
||
</p>
|
||
</th>
|
||
<th>
|
||
<p>
|
||
Semantics
|
||
</p>
|
||
</th>
|
||
</tr></thead>
|
||
<tbody>
|
||
<tr>
|
||
<td>
|
||
<p>
|
||
<code class="computeroutput"><span class="identifier">FA</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span></code>
|
||
</p>
|
||
</td>
|
||
<td>
|
||
<p>
|
||
creates an adapter, initializes the target function with <code class="computeroutput"><span class="identifier">f</span></code>.
|
||
</p>
|
||
</td>
|
||
</tr>
|
||
<tr>
|
||
<td>
|
||
<p>
|
||
<code class="computeroutput"><span class="identifier">FA</span><span class="special">()</span></code>
|
||
</p>
|
||
</td>
|
||
<td>
|
||
<p>
|
||
creates an adapter, attempts to use <code class="computeroutput"><span class="identifier">F</span></code>'s
|
||
default constructor.
|
||
</p>
|
||
</td>
|
||
</tr>
|
||
<tr>
|
||
<td>
|
||
<p>
|
||
<code class="computeroutput"><span class="identifier">fa</span><span class="special">(</span><span class="identifier">a0</span></code>...<code class="computeroutput"><span class="identifier">aN</span><span class="special">)</span></code>
|
||
</p>
|
||
</td>
|
||
<td>
|
||
<p>
|
||
calls <code class="computeroutput"><span class="identifier">f</span></code> with with
|
||
arguments <code class="computeroutput"><span class="identifier">a0</span></code>...<code class="computeroutput"><span class="identifier">aN</span></code>.
|
||
</p>
|
||
</td>
|
||
</tr>
|
||
</tbody>
|
||
</table></div>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.forward_adapter.h4"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.forward_adapter.limits"></a></span><a class="link" href="index.html#boost_functional_forward.reference.forward_adapter.limits">Limits</a>
|
||
</h5>
|
||
<p>
|
||
The macro BOOST_FUNCTIONAL_FORWARD_ADAPTER_MAX_ARITY can be defined to set
|
||
the maximum call arity. It defaults to 6.
|
||
</p>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.forward_adapter.h5"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.forward_adapter.complexity"></a></span><a class="link" href="index.html#boost_functional_forward.reference.forward_adapter.complexity">Complexity</a>
|
||
</h5>
|
||
<p>
|
||
Preprocessing time: O(2^N), where N is the arity limit. Compile time: O(2^N),
|
||
where N depends on the arity range. Run time: O(0) if the compiler inlines,
|
||
O(1) otherwise.
|
||
</p>
|
||
</div>
|
||
<div class="section">
|
||
<div class="titlepage"><div><div><h3 class="title">
|
||
<a name="boost_functional_forward.reference.lightweight_forward_adapter"></a><a class="link" href="index.html#boost_functional_forward.reference.lightweight_forward_adapter" title="lightweight_forward_adapter">lightweight_forward_adapter</a>
|
||
</h3></div></div></div>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.lightweight_forward_adapter.h0"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.lightweight_forward_adapter.description"></a></span><a class="link" href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.description">Description</a>
|
||
</h5>
|
||
<p>
|
||
Function object adapter template whose instances are callable with LValue
|
||
and RValue arguments. All arguments are forwarded as reference-to-const typed
|
||
LValues, except for reference wrappers which are unwrapped and may yield
|
||
non-const LValues.
|
||
</p>
|
||
<p>
|
||
An arity can be given as second, numeric non-type template argument to restrict
|
||
forwarding to a specific arity. If a third, numeric non-type template argument
|
||
is present, the second and third template argument are treated as minimum
|
||
and maximum arity, respectively. Specifying an arity can be helpful to improve
|
||
the readability of diagnostic messages and compile time performance.
|
||
</p>
|
||
<p>
|
||
<a href="http://www.boost.org/libs/utility/utility.htm#result_of" target="_top">Boost.ResultOf</a>
|
||
can be used to determine the result types of specific call expressions.
|
||
</p>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.lightweight_forward_adapter.h1"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.lightweight_forward_adapter.header"></a></span><a class="link" href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.header">Header</a>
|
||
</h5>
|
||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">functional</span><span class="special">/</span><span class="identifier">lightweight_forward_adapter</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||
</pre>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.lightweight_forward_adapter.h2"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.lightweight_forward_adapter.synopsis"></a></span><a class="link" href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.synopsis">Synopsis</a>
|
||
</h5>
|
||
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span>
|
||
<span class="special">{</span>
|
||
<span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Function</span><span class="special">,</span>
|
||
<span class="keyword">int</span> <span class="identifier">Arity_Or_MinArity</span> <span class="special">=</span> <span class="emphasis"><em>unspecified</em></span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">MaxArity</span> <span class="special">=</span> <span class="emphasis"><em>unspecified</em></span> <span class="special">></span>
|
||
<span class="keyword">struct</span> <span class="identifier">lightweight_forward_adapter</span><span class="special">;</span>
|
||
<span class="special">}</span>
|
||
</pre>
|
||
<div class="variablelist">
|
||
<p class="title"><b>Notation</b></p>
|
||
<dl class="variablelist">
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">F</span></code></span></dt>
|
||
<dd><p>
|
||
a possibly const qualified function object type or reference type thereof
|
||
</p></dd>
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">f</span></code></span></dt>
|
||
<dd><p>
|
||
an object convertible to <code class="computeroutput"><span class="identifier">F</span></code>
|
||
</p></dd>
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">FA</span></code></span></dt>
|
||
<dd><p>
|
||
the type <code class="computeroutput"><span class="identifier">lightweight_forward_adapter</span><span class="special"><</span><span class="identifier">F</span><span class="special">></span></code>
|
||
</p></dd>
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">fa</span></code></span></dt>
|
||
<dd><p>
|
||
an instance of <code class="computeroutput"><span class="identifier">FA</span></code>,
|
||
initialized with <code class="computeroutput"><span class="identifier">f</span></code>
|
||
</p></dd>
|
||
<dt><span class="term"><code class="computeroutput"><span class="identifier">a0</span></code>...<code class="computeroutput"><span class="identifier">aN</span></code></span></dt>
|
||
<dd><p>
|
||
arguments to <code class="computeroutput"><span class="identifier">fa</span></code>
|
||
</p></dd>
|
||
</dl>
|
||
</div>
|
||
<p>
|
||
The result type of a target function invocation must be
|
||
</p>
|
||
<pre class="programlisting"><a href="http://www.boost.org/libs/utility/utility.htm#result_of" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span></code></a><span class="special"><</span><span class="identifier">F</span><span class="special">*(</span><span class="identifier">TA0</span> <span class="special">[</span><span class="keyword">const</span><span class="special">]&...</span><span class="identifier">TAN</span> <span class="special">[</span><span class="keyword">const</span><span class="special">]&])>::</span><span class="identifier">type</span>
|
||
</pre>
|
||
<p>
|
||
where <code class="computeroutput"><span class="identifier">TA0</span></code>...<code class="computeroutput"><span class="identifier">TAN</span></code> denote the argument types of <code class="computeroutput"><span class="identifier">a0</span></code>...<code class="computeroutput"><span class="identifier">aN</span></code>.
|
||
</p>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.lightweight_forward_adapter.h3"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.lightweight_forward_adapter.expression_semantics"></a></span><a class="link" href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.expression_semantics">Expression
|
||
Semantics</a>
|
||
</h5>
|
||
<div class="informaltable"><table class="table">
|
||
<colgroup>
|
||
<col>
|
||
<col>
|
||
</colgroup>
|
||
<thead><tr>
|
||
<th>
|
||
<p>
|
||
Expression
|
||
</p>
|
||
</th>
|
||
<th>
|
||
<p>
|
||
Semantics
|
||
</p>
|
||
</th>
|
||
</tr></thead>
|
||
<tbody>
|
||
<tr>
|
||
<td>
|
||
<p>
|
||
<code class="computeroutput"><span class="identifier">FA</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span></code>
|
||
</p>
|
||
</td>
|
||
<td>
|
||
<p>
|
||
creates an adapter, initializes the target function with <code class="computeroutput"><span class="identifier">f</span></code>.
|
||
</p>
|
||
</td>
|
||
</tr>
|
||
<tr>
|
||
<td>
|
||
<p>
|
||
<code class="computeroutput"><span class="identifier">FA</span><span class="special">()</span></code>
|
||
</p>
|
||
</td>
|
||
<td>
|
||
<p>
|
||
creates an adapter, attempts to use <code class="computeroutput"><span class="identifier">F</span></code>'s
|
||
default constructor.
|
||
</p>
|
||
</td>
|
||
</tr>
|
||
<tr>
|
||
<td>
|
||
<p>
|
||
<code class="computeroutput"><span class="identifier">fa</span><span class="special">(</span><span class="identifier">a0</span></code>...<code class="computeroutput"><span class="identifier">aN</span><span class="special">)</span></code>
|
||
</p>
|
||
</td>
|
||
<td>
|
||
<p>
|
||
calls <code class="computeroutput"><span class="identifier">f</span></code> with with
|
||
const arguments <code class="computeroutput"><span class="identifier">a0</span></code>...<code class="computeroutput"><span class="identifier">aN</span></code>. If <code class="computeroutput"><span class="identifier">aI</span></code>
|
||
is a reference wrapper it is unwrapped.
|
||
</p>
|
||
</td>
|
||
</tr>
|
||
</tbody>
|
||
</table></div>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.lightweight_forward_adapter.h4"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.lightweight_forward_adapter.limits"></a></span><a class="link" href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.limits">Limits</a>
|
||
</h5>
|
||
<p>
|
||
The macro BOOST_FUNCTIONAL_LIGHTWEIGHT_FORWARD_ADAPTER_MAX_ARITY can be defined
|
||
to set the maximum call arity. It defaults to 10.
|
||
</p>
|
||
<h5>
|
||
<a name="boost_functional_forward.reference.lightweight_forward_adapter.h5"></a>
|
||
<span class="phrase"><a name="boost_functional_forward.reference.lightweight_forward_adapter.complexity"></a></span><a class="link" href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.complexity">Complexity</a>
|
||
</h5>
|
||
<p>
|
||
Preprocessing time: O(N), where N is the arity limit. Compile time: O(N),
|
||
where N is the effective arity of a call. Run time: O(0) if the compiler
|
||
inlines, O(1) otherwise.
|
||
</p>
|
||
</div>
|
||
</div>
|
||
<div class="section">
|
||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||
<a name="boost_functional_forward.acknowledgements"></a><a class="link" href="index.html#boost_functional_forward.acknowledgements" title="Acknowledgements">Acknowledgements</a>
|
||
</h2></div></div></div>
|
||
<p>
|
||
As these utilities are factored out of the <a href="http://www.boost.org/libs/fusion/doc/html/index.html" target="_top">Boost.Fusion</a>
|
||
functional module, I want to thank Dan Marsden and Joel de Guzman for letting
|
||
me participate in the development of that great library in the first place.
|
||
</p>
|
||
<p>
|
||
Further, I want to credit the authors of the references below, for their in-depth
|
||
investigation of the problem and the solution implemented here.
|
||
</p>
|
||
<p>
|
||
Last but not least I want to thank Vesa Karnoven and Paul Mensonides for the
|
||
Boost Preprocessor library. Without it, I would have ended up with an external
|
||
code generator for this one.
|
||
</p>
|
||
</div>
|
||
<div class="section">
|
||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||
<a name="boost_functional_forward.references"></a><a class="link" href="index.html#boost_functional_forward.references" title="References">References</a>
|
||
</h2></div></div></div>
|
||
<div class="orderedlist"><ol class="orderedlist" type="1">
|
||
<li class="listitem">
|
||
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm" target="_top">The
|
||
Forwarding Problem</a>, Peter Dimov, Howard E. Hinnant, David Abrahams,
|
||
2002
|
||
</li>
|
||
<li class="listitem">
|
||
<a href="http://www.boost.org/libs/utility/utility.htm#result_of" target="_top">Boost.ResultOf</a>,
|
||
Douglas Gregor, 2004
|
||
</li>
|
||
<li class="listitem">
|
||
<a href="http://www.boost.org/doc/html/ref.html" target="_top">Boost.Ref</a>,
|
||
Jaakko Jarvi, Peter Dimov, Douglas Gregor, David Abrahams, 1999-2002
|
||
</li>
|
||
</ol></div>
|
||
</div>
|
||
</div>
|
||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||
<td align="left"><p><small>Last revised: November 01, 2008 at 19:58:50 GMT</small></p></td>
|
||
<td align="right"><div class="copyright-footer"></div></td>
|
||
</tr></table>
|
||
<hr>
|
||
<div class="spirit-nav"></div>
|
||
</body>
|
||
</html>
|