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| <div><h2 class="title"> |
| <a name="boost_functional_forward"></a>Chapter 1. Boost.Functional/Forward 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><p class="copyright">Copyright © 2007, 2008 Tobias Schwinger</p></div> |
| <div><div class="legalnotice"> |
| <a name="id934161"></a><p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>) |
| </p> |
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| <div class="toc"> |
| <p><b>Table of Contents</b></p> |
| <dl> |
| <dt><span class="section"><a href="index.html#boost_functional_forward.brief_description">Brief Description</a></span></dt> |
| <dt><span class="section"><a href="index.html#boost_functional_forward.background">Background</a></span></dt> |
| <dt><span class="section"><a href="index.html#boost_functional_forward.reference"> Reference</a></span></dt> |
| <dt><span class="section"><a href="index.html#boost_functional_forward.acknowledgements">Acknowledgements</a></span></dt> |
| <dt><span class="section"><a href="index.html#boost_functional_forward.references">References</a></span></dt> |
| </dl> |
| </div> |
| <div class="section" lang="en"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_functional_forward.brief_description"></a><a href="index.html#boost_functional_forward.brief_description" title="Brief Description">Brief Description</a></h2></div></div></div> |
| <p> |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward_adapter</span></code> provides a reusable adapter |
| template for function objects. It forwards RValues as references to const, |
| while leaving LValues as-is. |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">g</span> <span class="comment">// function object that only accept LValues |
| </span><span class="special">{</span> |
| <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> |
| <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> |
| |
| <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// Adapted version also accepts RValues and forwards |
| </span><span class="comment">// them as references to const, LValues as-is |
| </span><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> |
| </pre> |
| <p> |
| Another adapter, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lighweight_forward_adapter</span></code> allows forwarding |
| with some help from the user accepting and unwrapping reference wrappers (see |
| <a href="http://www.boost.org/doc/html/ref.html" target="_top">Boost.Ref</a>) for |
| reference arguments, const qualifying all other arguments. |
| </p> |
| <p> |
| The target functions must be compatible with <a href="http://www.boost.org/libs/utility/utility.htm#result_of" target="_top">Boost.ResultOf</a>, |
| and so are the adapters. |
| </p> |
| </div> |
| <div class="section" lang="en"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_functional_forward.background"></a><a href="index.html#boost_functional_forward.background" title="Background">Background</a></h2></div></div></div> |
| <p> |
| Let's suppose we have some function <code class="computeroutput"><span class="identifier">f</span></code> |
| that we can call like this: |
| </p> |
| <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> |
| </pre> |
| <p> |
| Now we want to write another, generic function <code class="computeroutput"><span class="identifier">g</span></code> |
| 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. |
| </p> |
| <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> |
| </pre> |
| <a name="boost_functional_forward.background.why_would_we_want_to_do_it__anyway_"></a><h3> |
| <a name="id934701"></a> |
| <a href="index.html#boost_functional_forward.background.why_would_we_want_to_do_it__anyway_">Why |
| would we want to do it, anyway?</a> |
| </h3> |
| <p> |
| Maybe we want to run <code class="computeroutput"><span class="identifier">f</span></code> several |
| times. Or maybe we want to run it within another thread. Maybe we just want |
| to encapsulate the call expression for now, and then use it with other code |
| that allows to compose more complex expressions in order to decompose it with |
| C++ templates and have the compiler generate some machinery that eventually |
| calls <code class="computeroutput"><span class="identifier">f</span></code> at runtime (in other |
| words; apply a technique that is commonly referred to as Expression Templates). |
| </p> |
| <a name="boost_functional_forward.background.now__how_do_we_do_it_"></a><h3> |
| <a name="id934755"></a> |
| <a href="index.html#boost_functional_forward.background.now__how_do_we_do_it_">Now, |
| how do we do it?</a> |
| </h3> |
| <p> |
| The bad news is: It's impossible. |
| </p> |
| <p> |
| That is so because there is a slight difference between a variable and an expression |
| that evaluates to its value: Given |
| </p> |
| <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">y</span><span class="special">;</span> |
| <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> |
| </pre> |
| <p> |
| and |
| </p> |
| <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> |
| </pre> |
| <p> |
| we can call |
| </p> |
| <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><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></pre> |
| <p> |
| where |
| </p> |
| <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></pre> |
| <p> |
| This way we can safely have <code class="computeroutput"><span class="identifier">func1</span></code> |
| store its reference argument and the compiler keeps us from storing a reference |
| to an object with temporary lifetime. |
| </p> |
| <p> |
| It is important to realize that non-constness and whether an object binds to |
| a non-const reference parameter are two different properties. The latter is |
| the distinction between LValues and RValues. The names stem from the left hand |
| side and the right hand side of assignment expressions, thus LValues are typically |
| the ones you can assign to, and RValues the temporary results from the right |
| hand side expression. |
| </p> |
| <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><span class="comment">// 1+2 = a; // usually makes no sense. |
| </span> |
| <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><span class="comment">// func1(1+2); // fails to compile, because we only got an RValue. |
| </span></pre> |
| <p> |
| If we add const qualification on the parameter, our function also accepts RValues: |
| </p> |
| <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> |
| |
| <span class="comment">// [...] function scope: |
| </span><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><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><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></pre> |
| <p> |
| In all cases, the argument <code class="computeroutput"><span class="identifier">x</span></code> |
| in <code class="computeroutput"><span class="identifier">func2</span></code> is a const-qualified |
| LValue. We can use function overloading to identify non-const LValues: |
| </p> |
| <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><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> |
| <span class="comment">// [...] function scope: |
| </span><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><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><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></pre> |
| <p> |
| Note that all arguments <code class="computeroutput"><span class="identifier">x</span></code> in |
| the overloaded function <code class="computeroutput"><span class="identifier">func3</span></code> |
| are LValues. In fact, there is no way to transport RValues into a function |
| as-is in C++98. Also note that we can't distinguish between what used to be |
| a const qualified LValue and an RValue. |
| </p> |
| <p> |
| 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++ |
| 98. See <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm" target="_top">The |
| Forwarding Problem</a> for a very detailed discussion including solutions |
| that require language changes. |
| </p> |
| <p> |
| Now, for actually implementing it, we need 2^N overloads for N parameters (each |
| with and without const qualifier) for each number of arguments (that is 2^(Nmax+1) |
| - 2^Nmin). Right, that means the compile-time complexity is O(2^N), however |
| the factor is low so it works quite well for a reasonable number (< 10) |
| of arguments. |
| </p> |
| </div> |
| <div class="section" lang="en"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_functional_forward.reference"></a><a href="index.html#boost_functional_forward.reference" title=" Reference"> Reference</a></h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="index.html#boost_functional_forward.reference.forward_adapter">forward_adapter</a></span></dt> |
| <dt><span class="section"><a href="index.html#boost_functional_forward.reference.lightweight_forward_adapter">lightweight_forward_adapter</a></span></dt> |
| </dl></div> |
| <div class="section" lang="en"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_functional_forward.reference.forward_adapter"></a><a href="index.html#boost_functional_forward.reference.forward_adapter" title="forward_adapter">forward_adapter</a></h3></div></div></div> |
| <a name="boost_functional_forward.reference.forward_adapter.description"></a><h4> |
| <a name="id935594"></a> |
| <a href="index.html#boost_functional_forward.reference.forward_adapter.description">Description</a> |
| </h4> |
| <p> |
| Function object adapter template whose instances are callable with LValue |
| and RValue arguments. RValue arguments are forwarded as reference-to-const |
| typed 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> |
| <a name="boost_functional_forward.reference.forward_adapter.header"></a><h4> |
| <a name="id935646"></a> |
| <a href="index.html#boost_functional_forward.reference.forward_adapter.header">Header</a> |
| </h4> |
| <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> |
| <a name="boost_functional_forward.reference.forward_adapter.synopsis"></a><h4> |
| <a name="id935721"></a> |
| <a href="index.html#boost_functional_forward.reference.forward_adapter.synopsis">Synopsis</a> |
| </h4> |
| <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> |
| <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> |
| <a name="boost_functional_forward.reference.forward_adapter.expression_semantics"></a><h4> |
| <a name="id936177"></a> |
| <a href="index.html#boost_functional_forward.reference.forward_adapter.expression_semantics">Expression |
| Semantics</a> |
| </h4> |
| <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> |
| <a name="boost_functional_forward.reference.forward_adapter.limits"></a><h4> |
| <a name="id936405"></a> |
| <a href="index.html#boost_functional_forward.reference.forward_adapter.limits">Limits</a> |
| </h4> |
| <p> |
| The macro BOOST_FUNCTIONAL_FORWARD_ADAPTER_MAX_ARITY can be defined to set |
| the maximum call arity. It defaults to 6. |
| </p> |
| <a name="boost_functional_forward.reference.forward_adapter.complexity"></a><h4> |
| <a name="id936434"></a> |
| <a href="index.html#boost_functional_forward.reference.forward_adapter.complexity">Complexity</a> |
| </h4> |
| <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" lang="en"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_functional_forward.reference.lightweight_forward_adapter"></a><a href="index.html#boost_functional_forward.reference.lightweight_forward_adapter" title="lightweight_forward_adapter">lightweight_forward_adapter</a></h3></div></div></div> |
| <a name="boost_functional_forward.reference.lightweight_forward_adapter.description"></a><h4> |
| <a name="id936485"></a> |
| <a href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.description">Description</a> |
| </h4> |
| <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> |
| <a name="boost_functional_forward.reference.lightweight_forward_adapter.header"></a><h4> |
| <a name="id936540"></a> |
| <a href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.header">Header</a> |
| </h4> |
| <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> |
| <a name="boost_functional_forward.reference.lightweight_forward_adapter.synopsis"></a><h4> |
| <a name="id936616"></a> |
| <a href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.synopsis">Synopsis</a> |
| </h4> |
| <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> |
| <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> |
| <a name="boost_functional_forward.reference.lightweight_forward_adapter.expression_semantics"></a><h4> |
| <a name="id937073"></a> |
| <a href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.expression_semantics">Expression |
| Semantics</a> |
| </h4> |
| <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> |
| <a name="boost_functional_forward.reference.lightweight_forward_adapter.limits"></a><h4> |
| <a name="id937311"></a> |
| <a href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.limits">Limits</a> |
| </h4> |
| <p> |
| The macro BOOST_FUNCTIONAL_LIGHTWEIGHT_FORWARD_ADAPTER_MAX_ARITY can be defined |
| to set the maximum call arity. It defaults to 10. |
| </p> |
| <a name="boost_functional_forward.reference.lightweight_forward_adapter.complexity"></a><h4> |
| <a name="id937342"></a> |
| <a href="index.html#boost_functional_forward.reference.lightweight_forward_adapter.complexity">Complexity</a> |
| </h4> |
| <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" lang="en"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_functional_forward.acknowledgements"></a><a 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" lang="en"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_functional_forward.references"></a><a href="index.html#boost_functional_forward.references" title="References">References</a></h2></div></div></div> |
| <div class="orderedlist"><ol type="1"> |
| <li> |
| <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> |
| <a href="http://www.boost.org/libs/utility/utility.htm#result_of" target="_top">Boost.ResultOf</a>, |
| Douglas Gregor, 2004 |
| </li> |
| <li> |
| <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> |
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