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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="function.tutorial"></a>Tutorial</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#id-1.3.17.5.4">Basic Usage</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.17.5.5">Free functions</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.17.5.6">Member functions</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.17.5.7">References to Function Objects</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.17.5.8">Comparing Boost.Function function objects</a></span></dt>
</dl></div>
<p> Boost.Function has two syntactical forms: the preferred form
and the portable form. The preferred form fits more closely with the
C++ language and reduces the number of separate template parameters
that need to be considered, often improving readability; however, the
preferred form is not supported on all platforms due to compiler
bugs. The compatible form will work on all compilers supported by
Boost.Function. Consult the table below to determine which syntactic
form to use for your compiler.
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; ">
<li class="listitem">GNU C++ 2.95.x, 3.0.x and later versions</li>
<li class="listitem">Comeau C++ 4.2.45.2</li>
<li class="listitem">SGI MIPSpro 7.3.0</li>
<li class="listitem">Intel C++ 5.0, 6.0</li>
<li class="listitem">Compaq's cxx 6.2</li>
<li class="listitem">Microsoft Visual C++ 7.1 and later versions</li>
</ul></div>
</td>
<td align="left">
<div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; ">
<li class="listitem"><span class="emphasis"><em>Any compiler supporting the preferred syntax</em></span></li>
<li class="listitem">Microsoft Visual C++ 6.0, 7.0</li>
<li class="listitem">Borland C++ 5.5.1</li>
<li class="listitem">Sun WorkShop 6 update 2 C++ 5.3</li>
<li class="listitem">Metrowerks CodeWarrior 8.1</li>
</ul></div>
</td>
</tr></tbody>
</table></div>
<p>
</p>
<p> If your compiler does not appear in this list, please try the preferred syntax and report your results to the Boost list so that we can keep this table up-to-date.</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.17.5.4"></a>Basic Usage</h3></div></div></div>
<p> A function wrapper is defined simply
by instantiating the <code class="computeroutput">function</code> class
template with the desired return type and argument types, formulated
as a C++ function type. Any number of arguments may be supplied, up to
some implementation-defined limit (10 is the default maximum). The
following declares a function object wrapper
<code class="computeroutput">f</code> that takes two
<code class="computeroutput">int</code> parameters and returns a
<code class="computeroutput">float</code>:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><code class="computeroutput"><a class="link" href="../boost/function.html" title="Class template function">boost::function</a></code>&lt;float (int x, int y)&gt; f;</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><code class="computeroutput"><a class="link" href="../boost/functionN.html" title="Class template functionN">boost::function2</a></code>&lt;float, int, int&gt; f;</pre>
</td>
</tr></tbody>
</table></div>
<p>
</p>
<p> By default, function object wrappers are empty, so we can create a
function object to assign to <code class="computeroutput">f</code>:
</p>
<pre class="programlisting">struct int_div {
float operator()(int x, int y) const { return ((float)x)/y; };
};</pre>
<p>
</p>
<pre class="programlisting">f = int_div();</pre>
<p>
</p>
<p> Now we can use <code class="computeroutput">f</code> to execute
the underlying function object
<code class="computeroutput">int_div</code>:
</p>
<pre class="programlisting">std::cout &lt;&lt; f(5, 3) &lt;&lt; std::endl;</pre>
<p>
</p>
<p> We are free to assign any compatible function object to
<code class="computeroutput">f</code>. If
<code class="computeroutput">int_div</code> had been declared to take two
<code class="computeroutput">long</code> operands, the implicit
conversions would have been applied to the arguments without any user
interference. The only limit on the types of arguments is that they be
CopyConstructible, so we can even use references and arrays:
</p>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<thead><tr><th align="left">Preferred syntax</th></tr></thead>
<tbody><tr><td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><code class="computeroutput"><a class="link" href="../boost/function.html" title="Class template function">boost::function</a></code>&lt;void (int values[], int n, int&amp; sum, float&amp; avg)&gt; sum_avg;</pre>
</td></tr></tbody>
</table></div>
<p>
</p>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<thead><tr><th align="left">Portable syntax</th></tr></thead>
<tbody><tr><td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><code class="computeroutput"><a class="link" href="../boost/functionN.html" title="Class template functionN">boost::function4</a></code>&lt;void, int*, int, int&amp;, float&amp;&gt; sum_avg;</pre>
</td></tr></tbody>
</table></div>
<p>
</p>
<pre class="programlisting">void do_sum_avg(int values[], int n, int&amp; sum, float&amp; avg)
{
sum = 0;
for (int i = 0; i &lt; n; i++)
sum += values[i];
avg = (float)sum / n;
}</pre>
<p>
</p>
<pre class="programlisting">sum_avg = &amp;do_sum_avg;</pre>
<p>
</p>
<p> Invoking a function object wrapper that does not actually
contain a function object is a precondition violation, much like
trying to call through a null function pointer, and will throw a <code class="computeroutput"><a class="link" href="../boost/bad_function_call.html" title="Class bad_function_call">bad_function_call</a></code> exception). We can check for an
empty function object wrapper by using it in a boolean context (it evaluates <code class="computeroutput">true</code> if the wrapper is not empty) or compare it against <code class="computeroutput">0</code>. For instance:
</p>
<pre class="programlisting">if (f)
std::cout &lt;&lt; f(5, 3) &lt;&lt; std::endl;
else
std::cout &lt;&lt; "f has no target, so it is unsafe to call" &lt;&lt; std::endl;</pre>
<p>
</p>
<p> Alternatively,
<code class="computeroutput"><code class="computeroutput"><a class="link" href="../boost/function.html#id-1_3_17_6_2_1_4_28_1-bb">empty</a></code>()</code>
method will return whether or not the wrapper is empty. </p>
<p> Finally, we can clear out a function target by assigning it to <code class="computeroutput">0</code> or by calling the <code class="computeroutput"><code class="computeroutput"><a class="link" href="../boost/function.html#id-1_3_17_6_2_1_4_27_2-bb">clear</a></code>()</code> member function, e.g.,
</p>
<pre class="programlisting">f = 0;</pre>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.17.5.5"></a>Free functions</h3></div></div></div>
<p> Free function pointers can be considered singleton function objects with const function call operators, and can therefore be directly used with the function object wrappers:
</p>
<pre class="programlisting">float mul_ints(int x, int y) { return ((float)x) * y; }</pre>
<p>
</p>
<pre class="programlisting">f = &amp;mul_ints;</pre>
<p>
</p>
<p> Note that the <code class="computeroutput">&amp;</code> isn't really necessary unless you happen to be using Microsoft Visual C++ version 6. </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.17.5.6"></a>Member functions</h3></div></div></div>
<p> In many systems, callbacks often call to member functions of a
particular object. This is often referred to as "argument binding",
and is beyond the scope of Boost.Function. The use of member functions
directly, however, is supported, so the following code is valid:
</p>
<pre class="programlisting">struct X {
int foo(int);
};</pre>
<p>
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><code class="computeroutput"><a class="link" href="../boost/function.html" title="Class template function">boost::function</a></code>&lt;int (X*, int)&gt; f;
f = &amp;X::foo;
X x;
f(&amp;x, 5);</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><code class="computeroutput"><a class="link" href="../boost/functionN.html" title="Class template functionN">boost::function2</a></code>&lt;int, X*, int&gt; f;
f = &amp;X::foo;
X x;
f(&amp;x, 5);</pre>
</td>
</tr></tbody>
</table></div>
<p>
</p>
<p> Several libraries exist that support argument binding. Three such libraries are summarized below:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Bind. This library allows binding of
arguments for any function object. It is lightweight and very
portable.</p></li>
<li class="listitem">
<p>The C++ Standard library. Using
<code class="computeroutput">std::bind1st</code> and
<code class="computeroutput">std::mem_fun</code> together one can bind
the object of a pointer-to-member function for use with
Boost.Function:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"> <code class="computeroutput"><a class="link" href="../boost/function.html" title="Class template function">boost::function</a></code>&lt;int (int)&gt; f;
X x;
f = std::bind1st(
std::mem_fun(&amp;X::foo), &amp;x);
f(5); // Call x.foo(5)</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"> <code class="computeroutput"><a class="link" href="../boost/functionN.html" title="Class template functionN">boost::function1</a></code>&lt;int, int&gt; f;
X x;
f = std::bind1st(
std::mem_fun(&amp;X::foo), &amp;x);
f(5); // Call x.foo(5)</pre>
</td>
</tr></tbody>
</table></div>
<p>
</p>
</li>
<li class="listitem"><p>The <a class="link" href="../lambda.html" title="Chapter 20. Boost.Lambda">Lambda</a> library. This library provides a powerful composition mechanism to construct function objects that uses very natural C++ syntax. Lambda requires a compiler that is reasonably conformant to the C++ standard. </p></li>
</ul></div>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.17.5.7"></a>References to Function Objects</h3></div></div></div>
<p> In some cases it is
expensive (or semantically incorrect) to have Boost.Function clone a
function object. In such cases, it is possible to request that
Boost.Function keep only a reference to the actual function
object. This is done using the <code class="computeroutput">ref</code>
and <code class="computeroutput">cref</code> functions to wrap a
reference to a function object:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th align="left">Preferred syntax</th>
<th align="left">Portable syntax</th>
</tr></thead>
<tbody><tr>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">stateful_type a_function_object;
<code class="computeroutput"><a class="link" href="../boost/function.html" title="Class template function">boost::function</a></code>&lt;int (int)&gt; f;
f = <code class="computeroutput">boost::ref</code>(a_function_object);
<code class="computeroutput"><a class="link" href="../boost/function.html" title="Class template function">boost::function</a></code>&lt;int (int)&gt; f2(f);</pre>
</td>
<td align="left">
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">stateful_type a_function_object;
<code class="computeroutput"><a class="link" href="../boost/functionN.html" title="Class template functionN">boost::function1</a></code>&lt;int, int&gt; f;
f = <code class="computeroutput">boost::ref</code>(a_function_object);
<code class="computeroutput"><a class="link" href="../boost/functionN.html" title="Class template functionN">boost::function1</a></code>&lt;int, int&gt; f2(f);</pre>
</td>
</tr></tbody>
</table></div>
<p>
</p>
<p> Here, <code class="computeroutput">f</code> will not make a copy
of <code class="computeroutput">a_function_object</code>, nor will
<code class="computeroutput">f2</code> when it is targeted to
<code class="computeroutput">f</code>'s reference to
<code class="computeroutput">a_function_object</code>. Additionally, when
using references to function objects, Boost.Function will not throw
exceptions during assignment or construction.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.17.5.8"></a>Comparing Boost.Function function objects</h3></div></div></div>
<p>Function object wrappers can be compared via <code class="computeroutput">==</code>
or <code class="computeroutput">!=</code> against any function object that can be stored
within the wrapper. If the function object wrapper contains a
function object of that type, it will be compared against the given
function object (which must be either be
<a class="link" href="../EqualityComparable.html" title="Concept EqualityComparable">EqualityComparable</a> or have an overloaded <code class="computeroutput"><a class="link" href="../boost/function_equal.html" title="Function template function_equal">boost::function_equal</a></code>). For instance:</p>
<pre class="programlisting">int compute_with_X(X*, int);
f = &amp;X::foo;
assert(f == &amp;X::foo);
assert(&amp;compute_with_X != f);</pre>
<p>When comparing against an instance of
<code class="computeroutput">reference_wrapper</code>, the address
of the object in the
<code class="computeroutput">reference_wrapper</code> is compared
against the address of the object stored by the function object
wrapper:</p>
<pre class="programlisting">a_stateful_object so1, so2;
f = <code class="computeroutput">boost::ref</code>(so1);
assert(f == <code class="computeroutput">boost::ref</code>(so1));
assert(f == so1); <span class="emphasis"><em>// Only if a_stateful_object is <a class="link" href="../EqualityComparable.html" title="Concept EqualityComparable">EqualityComparable</a></em></span>
assert(f != <code class="computeroutput">boost::ref</code>(so2));</pre>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2001-2004 Douglas Gregor<p>Use, modification and distribution is subject to the Boost
Software License, Version 1.0. (See accompanying file
<code class="filename">LICENSE_1_0.txt</code> 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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