123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504505506507508509510511512513514515516517518519520521522523524525526527528529530531532533534535536537538539540541542543544545546547548549550551552553554555556557558559560561562563564565566567568569570571 |
- <html>
- <head>
- <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
- <title>Chapter 1. Boost.Functional/Forward 1.0</title>
- <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
- <meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
- <link rel="home" href="index.html" title="Chapter 1. Boost.Functional/Forward 1.0">
- </head>
- <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
- <table cellpadding="2" width="100%"><tr>
- <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
- <td align="center"><a href="../../../../../index.html">Home</a></td>
- <td align="center"><a href="../../../../libraries.htm">Libraries</a></td>
- <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
- <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
- <td align="center"><a href="../../../../../more/index.htm">More</a></td>
- </tr></table>
- <hr>
- <div class="spirit-nav"></div>
- <div class="chapter">
- <div class="titlepage"><div>
- <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="boost_functional_forward.legal"></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>
- </div></div>
- </div></div>
- <div class="toc">
- <p><b>Table of Contents</b></p>
- <dl class="toc">
- <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">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <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>
- </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">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="boost_functional_forward.background"></a><a class="link" 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>
- <h4>
- <a name="boost_functional_forward.background.h0"></a>
- <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
- would we want to do it, anyway?</a>
- </h4>
- <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>
- <h4>
- <a name="boost_functional_forward.background.h1"></a>
- <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
- do we do it?</a>
- </h4>
- <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">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <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>
- <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>
- <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">
- <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>
- </h3></div></div></div>
- <h5>
- <a name="boost_functional_forward.reference.forward_adapter.h0"></a>
- <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>
- <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>
- <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>
|