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- BoostBook element rationale"><link rel="refentry" href="Assignable.html" title="Concept Assignable"><link rel="refentry" href="InputIterator.html" title="Concept InputIterator"><link rel="refentry" href="OutputIterator.html" title="Concept OutputIterator"><link rel="refentry" href="ForwardIterator.html" title="Concept ForwardIterator"><link rel="refentry" href="BidirectionalIterator.html" title="Concept BidirectionalIterator"><link rel="refentry" href="RandomAccessIterator.html" title="Concept RandomAccessIterator"><link rel="refentry" href="DefaultConstructible.html" title="Concept DefaultConstructible"><link rel="refentry" href="CopyConstructible.html" title="Concept CopyConstructible"><link rel="refentry" href="EqualityComparable.html" title="Concept EqualityComparable"><link rel="refentry" href="LessThanComparable.html" title="Concept LessThanComparable"><link rel="refentry" href="SignedInteger.html" title="Concept SignedInteger"><link rel="subsection" href="function.tutorial.html#id2511232" title="Basic Usage"><link rel="subsection" href="function.tutorial.html#id2511507" title="Free functions"><link rel="subsection" href="function.tutorial.html#id2511537" title="Member functions"><link rel="subsection" href="function.tutorial.html#id2511686" title="References to Functions"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" border="1" cellpadding="2" bgcolor="#007F7F"><tr><td bgcolor="#FFFFFF"><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" width="277" height="86"></td><td><a href="../../index.htm"><font color="#FFFFFF" size="4" face="Arial">Home</font></a></td><td><a href="libraries.html"><font color="#FFFFFF" size="4" face="Arial">Libraries</font></a></td><td><a href="../../people/people.htm"><font color="#FFFFFF" size="4" face="Arial">People</font></a></td><td><a href="../../more/faq.htm"><font color="#FFFFFF" size="4" face="Arial">FAQ</font></a></td><td><a href="../../more/index.htm"><font color="#FFFFFF" size="4" face="Arial">More</font></a></td></tr></table><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="function.tutorial"></a>Tutorial</h3></div></div><div></div></div><div class="toc"><dl><dt><a href="function.tutorial.html#id2511232">Basic Usage</a></dt><dt><a href="function.tutorial.html#id2511507">Free functions</a></dt><dt><a href="function.tutorial.html#id2511537">Member functions</a></dt><dt><a href="function.tutorial.html#id2511686">References to Functions</a></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 border="1"><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 type="disc" compact><li>GNU C++ 2.95.x, 3.0.x, 3.1.x</li><li>Comeau C++ 4.2.45.2</li><li>SGI MIPSpro 7.3.0</li><li>Intel C++ 5.0, 6.0</li><li>Compaq's cxx 6.2</li><li>Microsoft Visual C++ 7.1</li></ul></div></td><td align="left"><div class="itemizedlist"><ul type="disc" compact><li><span class="emphasis"><em>Any compiler supporting the preferred syntax</em></span></li><li>Microsoft Visual C++ 6.0, 7.0</li><li>Borland C++ 5.5.1</li><li>Sun WorkShop 6 update 2 C++ 5.3</li><li>Metrowerks CodeWarrior 8.1</li></ul></div></td></tr></tbody></table></div><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" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2511232"></a>Basic Usage</h4></div></div><div></div></div><p> A function wrapper is defined simply
- by instantiating the <tt class="computeroutput">function</tt> 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
- <tt class="computeroutput">f</tt> that takes two
- <tt class="computeroutput">int</tt> parameters and returns a
- <tt class="computeroutput">float</tt>:
- </p><div class="informaltable"><table border="1"><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 class="programlisting"><a href="class.boost.function.html" title="Class template function">boost::function</a><float (int x, int y)> f;</pre></td><td align="left"><pre class="programlisting"><a href="class.boost.functionN.html" title="Class template functionN">boost::function2</a><float, int, int> f;</pre></td></tr></tbody></table></div><p> By default, function object wrappers are empty, so we can create a
- function object to assign to <tt class="computeroutput">f</tt>:
- </p><pre class="programlisting">struct int_div {
- float operator()(int x, int y) const { return ((float)x)/y; };
- };</pre><pre class="programlisting">f = int_div();</pre><p> Now we can use <tt class="computeroutput">f</tt> to execute
- the underlying function object
- <tt class="computeroutput">int_div</tt>:
- </p><pre class="programlisting">std::cout << f(5, 3) << std::endl;</pre><p> We are free to assign any compatible function object to
- <tt class="computeroutput">f</tt>. If
- <tt class="computeroutput">int_div</tt> had been declared to take two
- <tt class="computeroutput">long</tt> 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 border="1"><colgroup><col></colgroup><thead><tr><th align="left">Preferred syntax</th></tr></thead><tbody><tr><td align="left"><pre class="programlisting"><a href="class.boost.function.html" title="Class template function">boost::function</a><void (int values[], int n, int& sum, float& avg)> sum_avg;</pre></td></tr></tbody></table></div><div class="informaltable"><table border="1"><colgroup><col></colgroup><thead><tr><th align="left">Portable syntax</th></tr></thead><tbody><tr><td align="left"><pre class="programlisting"><a href="class.boost.functionN.html" title="Class template functionN">boost::function4</a><void, int*, int, int&, float&> sum_avg;</pre></td></tr></tbody></table></div><pre class="programlisting">void do_sum_avg(int values[], int n, int& sum, float& avg)
- {
- sum = 0;
- for (int i = 0; i < n; i++)
- sum += values[i];
- avg = (float)sum / n;
- }</pre><pre class="programlisting">sum_avg = &do_sum_avg;</pre><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 <a href="class.boost.bad_function_call.html" title="Class bad_function_call">bad_function_call</a> exception). We can check for an
- empty function object wrapper by using it in a boolean context (it evaluates <tt class="computeroutput">true</tt> if the wrapper is not empty) or compare it against <tt class="computeroutput">0</tt>. For instance:
- </p><pre class="programlisting">if (f)
- std::cout << f(5, 3) << std::endl;
- else
- std::cout << "f has no target, so it is unsafe to call" << std::endl;</pre><p> Alternatively,
- <tt class="computeroutput"><a href="class.boost.function.html#id2432398-bb">empty</a>()</tt>
- method will return whether or not the wrapper is empty. </p><p> Finally, we can clear out a function target by assigning it to <tt class="computeroutput">0</tt> or by calling the <tt class="computeroutput"><a href="class.boost.function.html#id2465894-bb">clear</a>()</tt> member function, e.g.,
- </p><pre class="programlisting">f = 0;</pre></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2511507"></a>Free functions</h4></div></div><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><pre class="programlisting">f = &mul_ints;</pre><p> Note that the <tt class="computeroutput">&</tt> isn't really necessary unless you happen to be using Microsoft Visual C++ version 6. </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2511537"></a>Member functions</h4></div></div><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><div class="informaltable"><table border="1"><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 class="programlisting"><a href="class.boost.function.html" title="Class template function">boost::function</a><int (X*, int)> f;
- f = &X::foo;
-
- X x;
- f(&x, 5);</pre></td><td align="left"><pre class="programlisting"><a href="class.boost.functionN.html" title="Class template functionN">boost::function2</a><int, X*, int> f;
- f = &X::foo;
-
- X x;
- f(&x, 5);</pre></td></tr></tbody></table></div><p> Several libraries exist that support argument binding. Three such libraries are summarized below:
- </p><div class="itemizedlist"><ul type="disc"><li><p><a href="../../libs/bind/index.html" target="_top">Bind</a>. This library allows binding of
- arguments for any function object. It is lightweight and very
- portable.</p></li><li><p>The C++ Standard library. Using
- <tt class="computeroutput">std::bind1st</tt> and
- <tt class="computeroutput">std::mem_fun</tt> together one can bind
- the object of a pointer-to-member function for use with
- Boost.Function:
- </p><div class="informaltable"><table border="1"><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 class="programlisting"><a href="class.boost.function.html" title="Class template function">boost::function</a><int (int)> f;
- X x;
- f = std::bind1st(
- std::mem_fun(&X::foo), &x);
- f(5); // Call x.foo(5)</pre></td><td align="left"><pre class="programlisting"><a href="class.boost.functionN.html" title="Class template functionN">boost::function1</a><int, int> f;
- X x;
- f = std::bind1st(
- std::mem_fun(&X::foo), &x);
- f(5); // Call x.foo(5)</pre></td></tr></tbody></table></div></li><li><p>The <a href="../../libs/lambda/index.html" target="_top">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></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2511686"></a>References to Functions</h4></div></div><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 <tt class="computeroutput">ref</tt>
- and <tt class="computeroutput">cref</tt> functions to wrap a
- reference to a function object:
- </p><div class="informaltable"><table border="1"><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 class="programlisting"> stateful_type a_function_object;
- <a href="class.boost.function.html" title="Class template function">boost::function</a><int (int)> f;
- f = <a href="class.boost.reference_wrapper.html#id2471260-bb">boost::ref</a>(a_function_object);
- <a href="class.boost.function.html" title="Class template function">boost::function</a><int (int)> f2(f);</pre></td><td align="left"><pre class="programlisting"> stateful_type a_function_object;
- <a href="class.boost.functionN.html" title="Class template functionN">boost::function1</a><int, int> f;
- f = <a href="class.boost.reference_wrapper.html#id2471260-bb">boost::ref</a>(a_function_object);
- <a href="class.boost.functionN.html" title="Class template functionN">boost::function1</a><int, int> f2(f);</pre></td></tr></tbody></table></div><p> Here, <tt class="computeroutput">f</tt> will not make a copy
- of <tt class="computeroutput">a_function_object</tt>, nor will
- <tt class="computeroutput">f2</tt> when it is targeted to
- <tt class="computeroutput">f</tt>'s reference to
- <tt class="computeroutput">a_function_object</tt>. Additionally, when
- using references to function objects, Boost.Function will not throw
- exceptions during assignment or construction.
- </p></div></div><table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr><td align="left"><small>Last revised: December 19, 2003 at 03:33:25 GMT</small></td><td align="right"><small>Copyright © 2001-2003 Douglas Gregor</small></td></tr></table><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="function.history.html"><img src="images/prev.png" alt="Prev"></a> </td><td width="20%" align="center"><a accesskey="u" href="function.html"><img src="images/up.png" alt="Up"></a></td><td width="40%" align="right"> <a accesskey="n" href="function.reference.html"><img src="images/next.png" alt="Next"></a></td></tr><tr><td width="40%" align="left" valign="top">History & Compatibility Notes </td><td width="20%" align="center"><a accesskey="h" href="index.html"><img src="images/home.png" alt="Home"></a></td><td width="40%" align="right" valign="top"> Reference</td></tr></table></div></body></html>
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