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<div><h2 class="title">
<a name="boost_functional_factory"></a>Chapter 1. Boost.Functional/Factory 1.0</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Tobias</span> <span class="surname">Schwinger</span>
</h3></div></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Glen</span> <span class="surname">Fernandes</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2007, 2008 Tobias Schwinger</p></div>
<div><p class="copyright">Copyright © 2019 Glen Joseph Fernandes</p></div>
<div><div class="legalnotice">
<a name="boost_functional_factory.legal"></a><p>
Distributed under the Boost Software License, Version 1.0.
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="index.html#boost_functional_factory.brief_description">Brief Description</a></span></dt>
<dt><span class="section"><a href="index.html#boost_functional_factory.background">Background</a></span></dt>
<dt><span class="section"><a href="index.html#boost_functional_factory.reference">Reference</a></span></dt>
<dt><span class="section"><a href="index.html#boost_functional_factory.changes">Changes</a></span></dt>
<dt><span class="section"><a href="index.html#boost_functional_factory.acknowledgements">Acknowledgements</a></span></dt>
<dt><span class="section"><a href="index.html#boost_functional_factory.references">References</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_functional_factory.brief_description"></a><a class="link" href="index.html#boost_functional_factory.brief_description" title="Brief Description">Brief Description</a>
</h2></div></div></div>
<p>
The template <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">factory</span></code> lets you encapsulate a <code class="computeroutput"><span class="keyword">new</span></code> expression as a function object, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">value_factory</span></code>
encapsulates a constructor invocation without <code class="computeroutput"><span class="keyword">new</span></code>.
</p>
<p>
</p>
<pre class="programlisting"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">factory</span></code><span class="special">&lt;</span><span class="identifier">T</span><span class="special">*&gt;()(</span><span class="identifier">arg1</span><span class="special">,</span><span class="identifier">arg2</span><span class="special">,</span><span class="identifier">arg3</span><span class="special">)</span>
<span class="comment">// same as new T(arg1,arg2,arg3)</span>
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">value_factory</span></code><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()(</span><span class="identifier">arg1</span><span class="special">,</span><span class="identifier">arg2</span><span class="special">,</span><span class="identifier">arg3</span><span class="special">)</span>
<span class="comment">// same as T(arg1,arg2,arg3)</span>
</pre>
<p>
</p>
<p>
Before C++11 the arguments to the function objects have to be LValues. A factory
that also accepts RValues can be composed using the <a href="http://www.boost.org/libs/functional/forward/doc/index.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward_adapter</span></code></a>
or <a href="http://www.boost.org/libs/bind/bind.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span></code></a>.
In C++11 or higher the arguments can be LValues or RValues.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_functional_factory.background"></a><a class="link" href="index.html#boost_functional_factory.background" title="Background">Background</a>
</h2></div></div></div>
<p>
In traditional Object Oriented Programming a Factory is an object implementing
an interface of one or more methods that construct objects conforming to known
interfaces.
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">// assuming a_concrete_class and another_concrete_class are derived</span>
<span class="comment">// from an_abstract_class</span>
<span class="keyword">struct</span> <span class="identifier">a_factory</span> <span class="special">{</span>
<span class="keyword">virtual</span> <span class="identifier">an_abstract_class</span><span class="special">*</span> <span class="identifier">create</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">a_factory</span><span class="special">()</span> <span class="special">{</span> <span class="special">}</span>
<span class="special">};</span>
<span class="keyword">struct</span> <span class="identifier">a_concrete_factory</span>
<span class="special">:</span> <span class="identifier">a_factory</span> <span class="special">{</span>
<span class="identifier">an_abstract_class</span><span class="special">*</span> <span class="identifier">create</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">a_concrete_class</span><span class="special">();</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="keyword">struct</span> <span class="identifier">another_concrete_factory</span>
<span class="special">:</span> <span class="identifier">a_factory</span> <span class="special">{</span>
<span class="identifier">an_abstract_class</span><span class="special">*</span> <span class="identifier">create</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">another_concrete_class</span><span class="special">();</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="comment">// [...]</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">ptr_map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">a_factory</span><span class="special">&gt;</span> <span class="identifier">factories</span><span class="special">;</span>
<span class="comment">// [...]</span>
<span class="identifier">factories</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="string">"a_name"</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">a_factory</span><span class="special">&gt;(</span><span class="keyword">new</span> <span class="identifier">a_concrete_factory</span><span class="special">));</span>
<span class="identifier">factories</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="string">"another_name"</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">a_factory</span><span class="special">&gt;(</span><span class="keyword">new</span> <span class="identifier">another_concrete_factory</span><span class="special">));</span>
<span class="comment">// [...]</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">an_abstract_class</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">factories</span><span class="special">.</span><span class="identifier">at</span><span class="special">(</span><span class="identifier">some_name</span><span class="special">).</span><span class="identifier">create</span><span class="special">());</span>
<span class="comment">// [...]</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
This approach has several drawbacks. The most obvious one is that there is
lots of boilerplate code. In other words there is too much code to express
a rather simple intention. We could use templates to get rid of some of it
but the approach remains inflexible:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
We may want a factory that takes some arguments that are forwarded to the
constructor,
</li>
<li class="listitem">
we will probably want to use smart pointers,
</li>
<li class="listitem">
we may want several member functions to create different kinds of objects,
</li>
<li class="listitem">
we might not necessarily need a polymorphic base class for the objects,
</li>
<li class="listitem">
as we will see, we do not need a factory base class at all,
</li>
<li class="listitem">
we might want to just call the constructor - without <code class="computeroutput"><span class="keyword">new</span></code>
to create an object on the stack, and
</li>
<li class="listitem">
finally we might want to use customized memory management.
</li>
</ul></div>
<p>
Experience has shown that using function objects and generic Boost components
for their composition, Design Patterns that describe callback mechanisms (typically
requiring a high percentage of boilerplate code with pure Object Oriented methodology)
become implementable with just few code lines and without extra classes.
</p>
<p>
Factories are callback mechanisms for constructors, so we provide two class
templates, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">value_factory</span></code> and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">factory</span></code>,
that encapsulate object construction via direct application of the constructor
and the <code class="computeroutput"><span class="keyword">new</span></code> operator, respectively.
</p>
<p>
We let the function objects forward their arguments to the construction expressions
they encapsulate. Over this <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">factory</span></code>
optionally allows the use of smart pointers and <a href="https://www.boost.org/sgi/stl/Allocators.html" target="_top">Allocators</a>.
</p>
<p>
Compile-time polymorphism can be used where appropriate,
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">do_something</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// [...]</span>
<span class="identifier">T</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">);</span>
<span class="comment">// for conceptually similar objects x we neither need virtual</span>
<span class="comment">// functions nor a common base class in this context.</span>
<span class="comment">// [...]</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
Now, to allow inhomogeneous signatures for the constructors of the types passed
in for <code class="computeroutput"><span class="identifier">T</span></code> we can use <code class="computeroutput"><span class="identifier">value_factory</span></code> and <a href="http://www.boost.org/libs/bind/bind.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span></code></a>
to normalize between them.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">ValueFactory</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">do_something</span><span class="special">(</span><span class="identifier">ValueFactory</span> <span class="identifier">make_obj</span> <span class="special">=</span> <span class="identifier">ValueFactory</span><span class="special">())</span>
<span class="special">{</span>
<span class="comment">// [...]</span>
<span class="keyword">typename</span> <span class="identifier">ValueFactory</span><span class="special">::</span><span class="identifier">result_type</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">make_obj</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">);</span>
<span class="comment">// for conceptually similar objects x we neither need virtual</span>
<span class="comment">// functions nor a common base class in this context.</span>
<span class="comment">// [...]</span>
<span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// [...]</span>
<span class="identifier">do_something</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">value_factory</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;());</span>
<span class="identifier">do_something</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">value_factory</span><span class="special">&lt;</span><span class="identifier">Y</span><span class="special">&gt;(),</span> <span class="identifier">_1</span><span class="special">,</span> <span class="number">5</span><span class="special">,</span> <span class="identifier">_2</span><span class="special">));</span>
<span class="comment">// construct X(a, b) and Y(a, 5, b), respectively.</span>
<span class="comment">// [...]</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
Maybe we want our objects to outlive the function's scope, in this case we
have to use dynamic allocation;
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Factory</span><span class="special">&gt;</span>
<span class="identifier">whatever</span> <span class="identifier">do_something</span><span class="special">(</span><span class="identifier">Factory</span> <span class="identifier">new_obj</span> <span class="special">=</span> <span class="identifier">Factory</span><span class="special">())</span>
<span class="special">{</span>
<span class="keyword">typename</span> <span class="identifier">Factory</span><span class="special">::</span><span class="identifier">result_type</span> <span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">new_obj</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">);</span>
<span class="comment">// again, no common base class or virtual functions needed,</span>
<span class="comment">// we could enforce a polymorphic base by writing e.g.</span>
<span class="comment">// boost::shared_ptr&lt;base&gt;</span>
<span class="comment">// instead of</span>
<span class="comment">// typename Factory::result_type</span>
<span class="comment">// above.</span>
<span class="comment">// Note that we are also free to have the type erasure happen</span>
<span class="comment">// somewhere else (e.g. in the constructor of this function's</span>
<span class="comment">// result type).</span>
<span class="comment">// [...]</span>
<span class="special">}</span>
<span class="comment">// [... call do_something like above but with boost::factory instead</span>
<span class="comment">// of boost::value_factory]</span>
</pre>
<p>
</p>
<p>
Although we might have created polymorphic objects in the previous example,
we have used compile time polymorphism for the factory. If we want to erase
the type of the factory and thus allow polymorphism at run time, we can use
<a href="http://www.boost.org/doc/html/function.html" target="_top">Boost.Function</a>
to do so. The first example can be rewritten as follows.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span><span class="identifier">an_abstract_class</span><span class="special">*()&gt;</span> <span class="identifier">a_factory</span><span class="special">;</span>
<span class="comment">// [...]</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">a_factory</span><span class="special">&gt;</span> <span class="identifier">factories</span><span class="special">;</span>
<span class="comment">// [...]</span>
<span class="identifier">factories</span><span class="special">[</span><span class="string">"a_name"</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">factory</span><span class="special">&lt;</span><span class="identifier">a_concrete_class</span><span class="special">*&gt;();</span>
<span class="identifier">factories</span><span class="special">[</span><span class="string">"another_name"</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">factory</span><span class="special">&lt;</span><span class="identifier">another_concrete_class</span><span class="special">*&gt;();</span>
<span class="comment">// [...]</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
Of course we can just as easy create factories that take arguments and/or return
<a href="http://www.boost.org/libs/smart_ptr/index.html" target="_top">Smart Pointers</a>.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_functional_factory.reference"></a><a class="link" href="index.html#boost_functional_factory.reference" title="Reference">Reference</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="index.html#boost_functional_factory.reference.value_factory">value_factory</a></span></dt>
<dt><span class="section"><a href="index.html#boost_functional_factory.reference.factory">factory</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_functional_factory.reference.value_factory"></a><a class="link" href="index.html#boost_functional_factory.reference.value_factory" title="value_factory">value_factory</a>
</h3></div></div></div>
<h5>
<a name="boost_functional_factory.reference.value_factory.h0"></a>
<span class="phrase"><a name="boost_functional_factory.reference.value_factory.description"></a></span><a class="link" href="index.html#boost_functional_factory.reference.value_factory.description">Description</a>
</h5>
<p>
Function object template that invokes the constructor of the type <code class="computeroutput"><span class="identifier">T</span></code>.
</p>
<h5>
<a name="boost_functional_factory.reference.value_factory.h1"></a>
<span class="phrase"><a name="boost_functional_factory.reference.value_factory.header"></a></span><a class="link" href="index.html#boost_functional_factory.reference.value_factory.header">Header</a>
</h5>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">functional</span><span class="special">/</span><span class="identifier">value_factory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
</p>
<h5>
<a name="boost_functional_factory.reference.value_factory.h2"></a>
<span class="phrase"><a name="boost_functional_factory.reference.value_factory.synopsis"></a></span><a class="link" href="index.html#boost_functional_factory.reference.value_factory.synopsis">Synopsis</a>
</h5>
<p>
</p>
<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">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">value_factory</span><span class="special">;</span>
<span class="special">}</span> <span class="comment">// boost</span>
</pre>
<p>
</p>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">T</span></code></span></dt>
<dd><p>
an arbitrary type with at least one public constructor
</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>
argument values to a constructor of <code class="computeroutput"><span class="identifier">T</span></code>
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">F</span></code></span></dt>
<dd><p>
the type <code class="computeroutput"><span class="identifier">value_factory</span><span class="special">&lt;</span><span class="identifier">F</span><span class="special">&gt;</span></code>
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">f</span></code></span></dt>
<dd><p>
an instance object of <code class="computeroutput"><span class="identifier">F</span></code>
</p></dd>
</dl>
</div>
<h5>
<a name="boost_functional_factory.reference.value_factory.h3"></a>
<span class="phrase"><a name="boost_functional_factory.reference.value_factory.expression_semantics"></a></span><a class="link" href="index.html#boost_functional_factory.reference.value_factory.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">F</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
creates an object of type <code class="computeroutput"><span class="identifier">F</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">F</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
creates an object of type <code class="computeroutput"><span class="identifier">F</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">f</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>
returns <code class="computeroutput"><span class="identifier">T</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>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">F</span><span class="special">::</span><span class="identifier">result_type</span></code>
</p>
</td>
<td>
<p>
is the type <code class="computeroutput"><span class="identifier">T</span></code>.
</p>
</td>
</tr>
</tbody>
</table></div>
<h5>
<a name="boost_functional_factory.reference.value_factory.h4"></a>
<span class="phrase"><a name="boost_functional_factory.reference.value_factory.limits"></a></span><a class="link" href="index.html#boost_functional_factory.reference.value_factory.limits">Limits</a>
</h5>
<p>
Before C++11, the maximum number of arguments supported is 10. Since C++11
an arbitrary number of arguments is supported.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_functional_factory.reference.factory"></a><a class="link" href="index.html#boost_functional_factory.reference.factory" title="factory">factory</a>
</h3></div></div></div>
<h5>
<a name="boost_functional_factory.reference.factory.h0"></a>
<span class="phrase"><a name="boost_functional_factory.reference.factory.description"></a></span><a class="link" href="index.html#boost_functional_factory.reference.factory.description">Description</a>
</h5>
<p>
Function object template that dynamically constructs a pointee object for
the type of pointer given as template argument. Smart pointers may be used
for the template argument, given that <code class="computeroutput"><span class="identifier">pointer_traits</span><span class="special">&lt;</span><span class="identifier">Pointer</span><span class="special">&gt;::</span><span class="identifier">element_type</span></code>
yields the pointee type.
</p>
<p>
If an <a href="https://www.boost.org/sgi/stl/Allocators.html" target="_top">Allocator</a>
is given, it is used for memory allocation and the placement form of the
<code class="computeroutput"><span class="keyword">new</span></code> operator is used to construct
the object. A function object that calls the destructor and deallocates the
memory with a copy of the Allocator is used for the second constructor argument
of <code class="computeroutput"><span class="identifier">Pointer</span></code> (thus it must
be a <a href="http://www.boost.org/libs/smart_ptr/index.html" target="_top">Smart Pointer</a>
that provides a suitable constructor, such as <a href="http://www.boost.org/libs/smart_ptr/shared_ptr.htm" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span></code></a>).
</p>
<p>
If a third template argument is <code class="computeroutput"><span class="identifier">factory_passes_alloc_to_smart_pointer</span></code>,
the allocator itself is used for the third constructor argument of <code class="computeroutput"><span class="identifier">Pointer</span></code> ( <a href="http://www.boost.org/libs/smart_ptr/shared_ptr.htm" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span></code></a> then uses the allocator
to manage the memory of its separately allocated reference counter).
</p>
<h5>
<a name="boost_functional_factory.reference.factory.h1"></a>
<span class="phrase"><a name="boost_functional_factory.reference.factory.header"></a></span><a class="link" href="index.html#boost_functional_factory.reference.factory.header">Header</a>
</h5>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">functional</span><span class="special">/</span><span class="identifier">factory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
</p>
<h5>
<a name="boost_functional_factory.reference.factory.h2"></a>
<span class="phrase"><a name="boost_functional_factory.reference.factory.synopsis"></a></span><a class="link" href="index.html#boost_functional_factory.reference.factory.synopsis">Synopsis</a>
</h5>
<p>
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
<span class="keyword">enum</span> <span class="identifier">factory_alloc_propagation</span> <span class="special">{</span>
<span class="identifier">factory_alloc_for_pointee_and_deleter</span><span class="special">,</span>
<span class="identifier">factory_passes_alloc_to_smart_pointer</span>
<span class="special">};</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Pointer</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">Allocator</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">,</span>
<span class="identifier">factory_alloc_propagation</span> <span class="identifier">Policy</span> <span class="special">=</span> <span class="identifier">factory_alloc_for_pointee_and_deleter</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">factory</span><span class="special">;</span>
<span class="special">}</span> <span class="comment">// boost</span>
</pre>
<p>
</p>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">T</span></code></span></dt>
<dd><p>
an arbitrary type with at least one public constructor
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">P</span></code></span></dt>
<dd><p>
pointer or smart pointer to <code class="computeroutput"><span class="identifier">T</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>
argument values to a constructor of <code class="computeroutput"><span class="identifier">T</span></code>
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">F</span></code></span></dt>
<dd><p>
the type <code class="computeroutput"><span class="identifier">factory</span><span class="special">&lt;</span><span class="identifier">P</span><span class="special">&gt;</span></code>
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">f</span></code></span></dt>
<dd><p>
an instance object of <code class="computeroutput"><span class="identifier">F</span></code>
</p></dd>
</dl>
</div>
<h5>
<a name="boost_functional_factory.reference.factory.h3"></a>
<span class="phrase"><a name="boost_functional_factory.reference.factory.expression_semantics"></a></span><a class="link" href="index.html#boost_functional_factory.reference.factory.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">F</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
creates an object of type <code class="computeroutput"><span class="identifier">F</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">F</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
creates an object of type <code class="computeroutput"><span class="identifier">F</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">f</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>
dynamically creates an object of type <code class="computeroutput"><span class="identifier">T</span></code>
using <code class="computeroutput"><span class="identifier">a0</span></code>...<code class="computeroutput"><span class="identifier">aN</span></code> as arguments for the constructor
invocation.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">F</span><span class="special">::</span><span class="identifier">result_type</span></code>
</p>
</td>
<td>
<p>
is the type <code class="computeroutput"><span class="identifier">P</span></code> with
top-level cv-qualifiers removed.
</p>
</td>
</tr>
</tbody>
</table></div>
<h5>
<a name="boost_functional_factory.reference.factory.h4"></a>
<span class="phrase"><a name="boost_functional_factory.reference.factory.limits"></a></span><a class="link" href="index.html#boost_functional_factory.reference.factory.limits">Limits</a>
</h5>
<p>
Before C++11, the maximum number of arguments supported is 10. Since C++11
an arbitrary number of arguments is supported.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_functional_factory.changes"></a><a class="link" href="index.html#boost_functional_factory.changes" title="Changes">Changes</a>
</h2></div></div></div>
<h4>
<a name="boost_functional_factory.changes.h0"></a>
<span class="phrase"><a name="boost_functional_factory.changes.boost_1_72_0"></a></span><a class="link" href="index.html#boost_functional_factory.changes.boost_1_72_0">Boost
1.72.0</a>
</h4>
<p>
Glen Fernandes rewrote the implementations of <code class="computeroutput"><span class="identifier">factory</span></code>
and <code class="computeroutput"><span class="identifier">value_factory</span></code> to provide
the following features:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Support r-value arguments when available
</li>
<li class="listitem">
Support arbitrary number of arguments via variadic templates when available
</li>
<li class="listitem">
Support allocators that are final
</li>
<li class="listitem">
Support allocators that use fancy pointers
</li>
<li class="listitem">
Support for disabled exceptions (<code class="computeroutput"><span class="identifier">BOOST_NO_EXCEPTIONS</span></code>)
</li>
<li class="listitem">
Improved compilation times
</li>
</ul></div>
<p>
The following features have been removed:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Increasing limits for C++03 compilers through <code class="computeroutput"><span class="identifier">BOOST_FUNCTIONAL_VALUE_FACTORY_MAX_ARITY</span></code>
</li>
<li class="listitem">
Using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code> in place of <code class="computeroutput"><span class="keyword">void</span></code>
through <code class="computeroutput"><span class="identifier">BOOST_FUNCTIONAL_FACTORY_SUPPORT_NONE_T</span></code>
</li>
</ul></div>
<h4>
<a name="boost_functional_factory.changes.h1"></a>
<span class="phrase"><a name="boost_functional_factory.changes.boost_1_58_0"></a></span><a class="link" href="index.html#boost_functional_factory.changes.boost_1_58_0">Boost
1.58.0</a>
</h4>
<p>
In order to remove the dependency on Boost.Optional, the default parameter
for allocators has been changed from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code>
to <code class="computeroutput"><span class="keyword">void</span></code>. If you have code that
has stopped working because it uses <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code>,
a quick fix is to define <code class="computeroutput"><span class="identifier">BOOST_FUNCTIONAL_FACTORY_SUPPORT_NONE_T</span></code>,
which will restore support, but this will be removed in a future release. It
should be be relatively easy to fix this properly.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_functional_factory.acknowledgements"></a><a class="link" href="index.html#boost_functional_factory.acknowledgements" title="Acknowledgements">Acknowledgements</a>
</h2></div></div></div>
<p>
Tobias Schwinger for creating this library.
</p>
<p>
Eric Niebler requested a function to invoke a type's constructor (with the
arguments supplied as a Tuple) as a Fusion feature. These Factory utilities
are a factored-out generalization of this idea.
</p>
<p>
Dave Abrahams suggested Smart Pointer support for exception safety, providing
useful hints for the implementation.
</p>
<p>
Joel de Guzman's documentation style was copied from Fusion.
</p>
<p>
Peter Dimov for sharing his insights on language details and their evolution.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_functional_factory.references"></a><a class="link" href="index.html#boost_functional_factory.references" title="References">References</a>
</h2></div></div></div>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<a href="http://en.wikipedia.org/wiki/Design_Patterns" target="_top">Design Patterns</a>,
Gamma et al. - Addison Wesley Publishing, 1995
</li>
<li class="listitem">
<a href="https://boost.org/sgi/stl/" target="_top">Standard Template Library Programmer's
Guide</a>, Hewlett-Packard Company, 1994
</li>
<li class="listitem">
<a href="http://www.boost.org/libs/bind/bind.html" target="_top">Boost.Bind</a>,
Peter Dimov, 2001-2005
</li>
<li class="listitem">
<a href="http://www.boost.org/doc/html/function.html" target="_top">Boost.Function</a>,
Douglas Gregor, 2001-2004
</li>
</ol></div>
</div>
</div>
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