1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Tutorial</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="The Boost C++ Libraries BoostBook Documentation Subset">
<link rel="up" href="../function.html" title="Chapter 15. Boost.Function">
<link rel="prev" href="history.html" title="History & Compatibility Notes">
<link rel="next" href="reference.html" title="Reference">
</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="../../../libs/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">
<a accesskey="p" href="history.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../function.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
<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-1.3.16.5.4">Basic Usage</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.16.5.5">Free functions</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.16.5.6">Member functions</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.16.5.7">References to Function Objects</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.16.5.8">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-1.3.16.5.4"></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><float (int x, int y)> 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><float, int, int> 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 << f(5, 3) << 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><void (int values[], int n, int& sum, float& avg)> 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><void, int*, int, int&, float&> sum_avg;</pre>
</td></tr></tbody>
</table></div>
<p>
</p>
<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>
<p>
</p>
<pre class="programlisting">sum_avg = &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 << f(5, 3) << std::endl;
else
std::cout << "f has no target, so it is unsafe to call" << std::endl;</pre>
<p>
</p>
<p> Alternatively,
<code class="computeroutput"><code class="computeroutput"><a class="link" href="../boost/function.html#id-1_3_16_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_16_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-1.3.16.5.5"></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 = &mul_ints;</pre>
<p>
</p>
<p> Note that the <code class="computeroutput">&</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-1.3.16.5.6"></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><int (X*, int)> f;
f = &X::foo;
X x;
f(&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><int, X*, int> f;
f = &X::foo;
X x;
f(&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><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 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><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>
<p>
</p>
</li>
<li class="listitem"><p>The <a class="link" href="../lambda.html" title="Chapter 19. 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-1.3.16.5.7"></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><int (int)> 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><int (int)> 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><int, int> 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><int, int> 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-1.3.16.5.8"></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 = &X::foo;
assert(f == &X::foo);
assert(&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>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="history.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../function.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
|
