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
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
|
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Concepts in Depth</title>
<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
<link rel="up" href="../boost_typeerasure.html" title="Chapter 33. Boost.TypeErasure">
<link rel="prev" href="multi.html" title="Functions with Multiple Arguments">
<link rel="next" href="any.html" title="Using Any">
</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="multi.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_typeerasure.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="any.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="boost_typeerasure.concept"></a><a class="link" href="concept.html" title="Concepts in Depth">Concepts in Depth</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="concept.html#boost_typeerasure.concept.custom">Defining Custom Concepts</a></span></dt>
<dt><span class="section"><a href="concept.html#boost_typeerasure.concept.overload">Overloading</a></span></dt>
<dt><span class="section"><a href="concept.html#boost_typeerasure.concept.concept_map">Concept Maps</a></span></dt>
<dt><span class="section"><a href="concept.html#boost_typeerasure.concept.overload0">Associated Types</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typeerasure.concept.custom"></a><a class="link" href="concept.html#boost_typeerasure.concept.custom" title="Defining Custom Concepts">Defining Custom Concepts</a>
</h3></div></div></div>
<p>
(For the source of the examples in this section see <a href="../../../libs/type_erasure/example/custom.cpp" target="_top">custom.cpp</a>)
</p>
<p>
Earlier, we used <code class="computeroutput"><a class="link" href="../BOOST_TYPE_ERASURE_MEMBER.html" title="Macro BOOST_TYPE_ERASURE_MEMBER">BOOST_TYPE_ERASURE_MEMBER</a></code>
to define a concept for containers that support <code class="computeroutput"><span class="identifier">push_back</span></code>.
Sometimes this interface isn't flexible enough, however. The library also
provides a lower level interface that gives full control of the behavior.
Let's take a look at what we would need in order to define <code class="computeroutput"><span class="identifier">has_push_back</span><span class="special">.</span></code>
First, we need to define the <code class="computeroutput"><span class="identifier">has_push_back</span></code>
template itself. We'll give it two template parameters, one for the container
and one for the element type. This template must have a static member function
called apply which is used to execute the operation.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">C</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">has_push_back</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="identifier">C</span><span class="special">&</span> <span class="identifier">cont</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">cont</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">arg</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Now, we can use this in an <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
using <code class="computeroutput"><a class="link" href="../boost/type_erasure/call_idp150247888.html" title="Function call">call</a></code> to
dispatch the operation.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">has_push_back</span><span class="special"><</span><span class="identifier">_self</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">c</span><span class="special">(</span><span class="identifier">vec</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="identifier">call</span><span class="special">(</span><span class="identifier">has_push_back</span><span class="special"><</span><span class="identifier">_self</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>(),</span> <span class="identifier">c</span><span class="special">,</span> <span class="identifier">i</span><span class="special">);</span>
<span class="comment">// vec is [10].</span>
</pre>
<p>
</p>
<p>
Our second task is to customize <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
so that we can call <code class="computeroutput"><span class="identifier">c</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">10</span><span class="special">)</span></code>. We do
this by specializing <code class="computeroutput"><a class="link" href="../boost/type_erasure/concept_interface.html" title="Struct template concept_interface">concept_interface</a></code>.
The first argument is <code class="computeroutput"><span class="identifier">has_push_back</span></code>,
since we want to inject a member into every <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
that uses the <code class="computeroutput"><span class="identifier">has_push_back</span></code>
concept. The second argument, <code class="computeroutput"><span class="identifier">Base</span></code>,
is used by the library to chain multiple uses of <code class="computeroutput"><a class="link" href="../boost/type_erasure/concept_interface.html" title="Struct template concept_interface">concept_interface</a></code>
together. We have to inherit from it publicly. <code class="computeroutput"><span class="identifier">Base</span></code>
is also used to get access to the full <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
type. The third argument is the placeholder that represents this any. If
someone used <code class="computeroutput"><span class="identifier">push_back</span><span class="special"><</span><span class="identifier">_c</span><span class="special">,</span> <span class="identifier">_b</span><span class="special">></span></code>, we only want to insert a <code class="computeroutput"><span class="identifier">push_back</span></code> member in the container, not
the value type. Thus, the third argument is the container placeholder.
</p>
<p>
When we define <code class="computeroutput"><span class="identifier">push_back</span></code>
the argument type uses the metafunction <code class="computeroutput"><a class="link" href="../boost/type_erasure/as_param.html" title="Struct template as_param">as_param</a></code>.
This is just to handle the case where <code class="computeroutput"><span class="identifier">T</span></code>
is a placeholder. If <code class="computeroutput"><span class="identifier">T</span></code> is
not a placeholder, then the metafunction just returns its argument, <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span></code>, unchanged.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
<span class="keyword">namespace</span> <span class="identifier">type_erasure</span> <span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">C</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Base</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">concept_interface</span><span class="special"><</span><span class="identifier">has_push_back</span><span class="special"><</span><span class="identifier">C</span><span class="special">,</span> <span class="identifier">T</span><span class="special">>,</span> <span class="identifier">Base</span><span class="special">,</span> <span class="identifier">C</span><span class="special">></span> <span class="special">:</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="keyword">void</span> <span class="identifier">push_back</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">as_param</span><span class="special"><</span><span class="identifier">Base</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span><span class="special">&>::</span><span class="identifier">type</span> <span class="identifier">arg</span><span class="special">)</span>
<span class="special">{</span> <span class="identifier">call</span><span class="special">(</span><span class="identifier">has_push_back</span><span class="special"><</span><span class="identifier">C</span><span class="special">,</span> <span class="identifier">T</span><span class="special">>(),</span> <span class="special">*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">arg</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
Our example now becomes
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">has_push_back</span><span class="special"><</span><span class="identifier">_self</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">c</span><span class="special">(</span><span class="identifier">vec</span><span class="special">);</span>
<span class="identifier">c</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
which is what we want.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typeerasure.concept.overload"></a><a class="link" href="concept.html#boost_typeerasure.concept.overload" title="Overloading">Overloading</a>
</h3></div></div></div>
<p>
(For the source of the examples in this section see <a href="../../../libs/type_erasure/example/overload.cpp" target="_top">overload.cpp</a>)
</p>
<p>
<code class="computeroutput"><a class="link" href="../boost/type_erasure/concept_interface.html" title="Struct template concept_interface">concept_interface</a></code>
allows us to inject arbitrary declarations into an <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>.
This is very flexible, but there are some pitfalls to watch out for. Sometimes
we want to use the same concept several times with different parameters.
Specializing <code class="computeroutput"><a class="link" href="../boost/type_erasure/concept_interface.html" title="Struct template concept_interface">concept_interface</a></code>
in a way that handles overloads correctly is a bit tricky. Given a concept
foo, we'd like the following to work:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">any</span><span class="special"><</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span>
<span class="identifier">foo</span><span class="special"><</span><span class="identifier">_self</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>,</span>
<span class="identifier">foo</span><span class="special"><</span><span class="identifier">_self</span><span class="special">,</span> <span class="keyword">double</span><span class="special">>,</span>
<span class="identifier">copy_constructible</span><span class="special"><></span>
<span class="special">></span>
<span class="special">></span> <span class="identifier">x</span> <span class="special">=</span> <span class="special">...;</span>
<span class="identifier">x</span><span class="special">.</span><span class="identifier">foo</span><span class="special">(</span><span class="number">1</span><span class="special">);</span> <span class="comment">// calls foo(int)</span>
<span class="identifier">x</span><span class="special">.</span><span class="identifier">foo</span><span class="special">(</span><span class="number">1.0</span><span class="special">);</span> <span class="comment">// calls foo(double)</span>
</pre>
<p>
</p>
<p>
Because <code class="computeroutput"><a class="link" href="../boost/type_erasure/concept_interface.html" title="Struct template concept_interface">concept_interface</a></code>
creates a linear inheritance chain, without some extra work, one overload
of foo will hide the other.
</p>
<p>
Here are the techniques that I found work reliably.
</p>
<p>
For member functions I couldn't find a way to avoid using two specializations.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">foo</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">t</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&</span> <span class="identifier">u</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">t</span><span class="special">.</span><span class="identifier">foo</span><span class="special">(</span><span class="identifier">u</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
<span class="keyword">namespace</span> <span class="identifier">type_erasure</span> <span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Base</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Enable</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">concept_interface</span><span class="special"><</span> <span class="special">::</span><span class="identifier">foo</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>,</span> <span class="identifier">Base</span><span class="special">,</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">Enable</span><span class="special">></span> <span class="special">:</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">_fun_defined</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">as_param</span><span class="special"><</span><span class="identifier">Base</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&>::</span><span class="identifier">type</span> <span class="identifier">arg</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">call</span><span class="special">(::</span><span class="identifier">foo</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>(),</span> <span class="special">*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">arg</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Base</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">concept_interface</span><span class="special"><</span> <span class="special">::</span><span class="identifier">foo</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>,</span> <span class="identifier">Base</span><span class="special">,</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Base</span><span class="special">::</span><span class="identifier">_fun_defined</span><span class="special">></span> <span class="special">:</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="keyword">using</span> <span class="identifier">Base</span><span class="special">::</span><span class="identifier">foo</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">as_param</span><span class="special"><</span><span class="identifier">Base</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&>::</span><span class="identifier">type</span> <span class="identifier">arg</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">call</span><span class="special">(::</span><span class="identifier">foo</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>(),</span> <span class="special">*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">arg</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
This uses SFINAE to detect whether a using declaration is needed. Note that
the fourth argument of <code class="computeroutput"><a class="link" href="../boost/type_erasure/concept_interface.html" title="Struct template concept_interface">concept_interface</a></code>
is a dummy parameter which is always void and is intended to be used for
SFINAE. Another solution to the problem that I've used in the past is to
inject a dummy declaration of <code class="computeroutput"><span class="identifier">fun</span></code>
and always put in a using declaration. This is an inferior solution for several
reasons. It requires an extra interface to add the dummy overload. It also
means that <code class="computeroutput"><span class="identifier">fun</span></code> is always
overloaded, even if the user only asked for one overload. This makes it harder
to take the address of fun.
</p>
<p>
Note that while using SFINAE requires some code to be duplicated, the amount
of code that has to be duplicated is relatively small, since the implementation
of <code class="computeroutput"><a class="link" href="../boost/type_erasure/concept_interface.html" title="Struct template concept_interface">concept_interface</a></code>
is usually a one liner. It's a bit annoying, but I believe it's an acceptable
cost in lieu of a better solution.
</p>
<p>
For free functions you can use inline friends.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">bar_concept</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">t</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&</span> <span class="identifier">u</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">bar</span><span class="special">(</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">u</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
<span class="keyword">namespace</span> <span class="identifier">type_erasure</span> <span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Base</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">concept_interface</span><span class="special"><</span> <span class="special">::</span><span class="identifier">bar_concept</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>,</span> <span class="identifier">Base</span><span class="special">,</span> <span class="identifier">T</span><span class="special">></span> <span class="special">:</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="keyword">friend</span> <span class="keyword">void</span> <span class="identifier">bar</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">derived</span><span class="special"><</span><span class="identifier">Base</span><span class="special">>::</span><span class="identifier">type</span><span class="special">&</span> <span class="identifier">t</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">as_param</span><span class="special"><</span><span class="identifier">Base</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&>::</span><span class="identifier">type</span> <span class="identifier">u</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">call</span><span class="special">(::</span><span class="identifier">bar_concept</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>(),</span> <span class="identifier">t</span><span class="special">,</span> <span class="identifier">u</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Base</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">concept_interface</span><span class="special"><</span> <span class="special">::</span><span class="identifier">bar_concept</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>,</span> <span class="identifier">Base</span><span class="special">,</span> <span class="identifier">U</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">disable_if</span><span class="special"><</span><span class="identifier">is_placeholder</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="special">:</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="keyword">using</span> <span class="identifier">Base</span><span class="special">::</span><span class="identifier">bar</span><span class="special">;</span>
<span class="keyword">friend</span> <span class="keyword">void</span> <span class="identifier">bar</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">t</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">typename</span> <span class="identifier">derived</span><span class="special"><</span><span class="identifier">Base</span><span class="special">>::</span><span class="identifier">type</span><span class="special">&</span> <span class="identifier">u</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">call</span><span class="special">(::</span><span class="identifier">bar_concept</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>(),</span> <span class="identifier">t</span><span class="special">,</span> <span class="identifier">u</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
Basically we have to specialize <code class="computeroutput"><a class="link" href="../boost/type_erasure/concept_interface.html" title="Struct template concept_interface">concept_interface</a></code>
once for each argument to make sure that an overload is injected into the
first argument that's a placeholder. As you might have noticed, the argument
types are a bit tricky. In the first specialization, the first argument uses
<code class="computeroutput"><a class="link" href="../boost/type_erasure/derived.html" title="Struct template derived">derived</a></code> instead
of <code class="computeroutput"><a class="link" href="../boost/type_erasure/as_param.html" title="Struct template as_param">as_param</a></code>. The
reason for this is that if we used <code class="computeroutput"><a class="link" href="../boost/type_erasure/as_param.html" title="Struct template as_param">as_param</a></code>,
then we could end up violating the one definition rule by defining the same
function twice. Similarly, we use SFINAE in the second specialization to
make sure that bar is only defined once when both arguments are placeholders.
It's possible to merge the two specializations with a bit of metaprogramming,
but unless you have a lot of arguments, it's probably not worth while.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typeerasure.concept.concept_map"></a><a class="link" href="concept.html#boost_typeerasure.concept.concept_map" title="Concept Maps">Concept Maps</a>
</h3></div></div></div>
<p>
(For the source of the examples in this section see <a href="../../../libs/type_erasure/example/concept_map.cpp" target="_top">concept_map.cpp</a>)
</p>
<p>
Sometimes it is useful to non-intrusively adapt a type to model a concept.
For example, suppose that we want to make <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">type_info</span></code>
model <code class="computeroutput"><a class="link" href="../boost/type_erasure/less_than_comparable.html" title="Struct template less_than_comparable">less_than_comparable</a></code>.
To do this, we simply specialize the concept definition.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
<span class="keyword">namespace</span> <span class="identifier">type_erasure</span> <span class="special">{</span>
<span class="keyword">template</span><span class="special"><></span>
<span class="keyword">struct</span> <span class="identifier">less_than_comparable</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">type_info</span><span class="special">></span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">bool</span> <span class="identifier">apply</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">type_info</span><span class="special">&</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">type_info</span><span class="special">&</span> <span class="identifier">rhs</span><span class="special">)</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">lhs</span><span class="special">.</span><span class="identifier">before</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span> <span class="special">!=</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span>
<span class="special">};</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Most, but not all of the builtin concepts can be specialized. Constructors,
destructors, and RTTI need special treatment from the library and cannot
be specialized. Only primitive concepts can be specialized, so the iterator
concepts are also out.
</p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typeerasure.concept.overload0"></a><a class="link" href="concept.html#boost_typeerasure.concept.overload0" title="Associated Types">Associated Types</a>
</h3></div></div></div>
<p>
(For the source of the examples in this section see <a href="../../../libs/type_erasure/example/associated.cpp" target="_top">associated.cpp</a>)
</p>
<p>
Associated types such as <code class="computeroutput"><span class="keyword">typename</span>
<span class="identifier">T</span><span class="special">::</span><span class="identifier">value_type</span></code> or <code class="computeroutput"><span class="keyword">typename</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">iterator_traits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference</span></code> are quite common in template
programming. Boost.TypeErasure handles them using the <code class="computeroutput"><a class="link" href="../boost/type_erasure/deduced.html" title="Struct template deduced">deduced</a></code>
template. <code class="computeroutput"><a class="link" href="../boost/type_erasure/deduced.html" title="Struct template deduced">deduced</a></code>
is just like an ordinary <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>,
except that the type that it binds to is determined by calling a metafunction
and does not need to be specified explicitly.
</p>
<p>
For example, we can define a concept for holding an iterator, raw pointer,
or smart pointer as follows. First, we define a metafunction called <code class="computeroutput"><span class="identifier">pointee</span></code> defining the associated type.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">pointee</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">eval_if</span><span class="special"><</span><span class="identifier">is_placeholder</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">identity</span><span class="special"><</span><span class="keyword">void</span><span class="special">>,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">pointee</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span>
<span class="special">>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Note that we can't just use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">pointee</span></code>,
because this metafunction needs to be safe to instantiate with placeholders.
It doesn't matter what it returns as long as it doesn't give an error. (The
library never tries to instantiate it with a placeholder, but argument dependent
lookup can cause spurious instantiations.)
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span> <span class="special">=</span> <span class="identifier">_self</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">pointer</span> <span class="special">:</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span>
<span class="identifier">copy_constructible</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span>
<span class="identifier">dereferenceable</span><span class="special"><</span><span class="identifier">deduced</span><span class="special"><</span><span class="identifier">pointee</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">>&,</span> <span class="identifier">T</span><span class="special">></span>
<span class="special">></span>
<span class="special">{</span>
<span class="comment">// provide a typedef for convenience</span>
<span class="keyword">typedef</span> <span class="identifier">deduced</span><span class="special"><</span><span class="identifier">pointee</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">></span> <span class="identifier">element_type</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Now the Concept of <code class="computeroutput"><span class="identifier">x</span></code> uses
two placeholders, <code class="computeroutput"><span class="identifier">_self</span></code> and
<code class="computeroutput"><span class="identifier">pointer</span><span class="special"><>::</span><span class="identifier">element_type</span></code>. When we construct <code class="computeroutput"><span class="identifier">x</span></code>, with an <code class="computeroutput"><span class="keyword">int</span><span class="special">*</span></code>, <code class="computeroutput"><span class="identifier">pointer</span><span class="special"><>::</span><span class="identifier">element_type</span></code>
is deduced as <code class="computeroutput"><span class="identifier">pointee</span><span class="special"><</span><span class="keyword">int</span><span class="special">*>::</span><span class="identifier">type</span></code> which is <code class="computeroutput"><span class="keyword">int</span></code>.
Thus, dereferencing <code class="computeroutput"><span class="identifier">x</span></code> returns
an <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> that contains
an <code class="computeroutput"><span class="keyword">int</span></code>.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span>
<span class="identifier">pointer</span><span class="special"><>,</span>
<span class="identifier">typeid_</span><span class="special"><</span><span class="identifier">pointer</span><span class="special"><>::</span><span class="identifier">element_type</span><span class="special">></span>
<span class="special">></span>
<span class="special">></span> <span class="identifier">x</span><span class="special">(&</span><span class="identifier">i</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">any_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(*</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// j == i</span>
</pre>
<p>
</p>
<p>
Sometimes we want to require that the associated type be a specific type.
This can be solved using the <code class="computeroutput"><a class="link" href="../boost/type_erasure/same_type.html" title="Struct template same_type">same_type</a></code>
concept. Here we create an any that can hold any pointer whose element type
is <code class="computeroutput"><span class="keyword">int</span></code>.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span>
<span class="identifier">pointer</span><span class="special"><>,</span>
<span class="identifier">same_type</span><span class="special"><</span><span class="identifier">pointer</span><span class="special"><>::</span><span class="identifier">element_type</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span>
<span class="special">></span>
<span class="special">></span> <span class="identifier">x</span><span class="special">(&</span><span class="identifier">i</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">*</span><span class="identifier">x</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 10</span>
</pre>
<p>
</p>
<p>
Using <code class="computeroutput"><a class="link" href="../boost/type_erasure/same_type.html" title="Struct template same_type">same_type</a></code>
like this effectively causes the library to replace all uses of <code class="computeroutput"><span class="identifier">pointer</span><span class="special"><>::</span><span class="identifier">element_type</span></code> with <code class="computeroutput"><span class="keyword">int</span></code>
and validate that it is always bound to <code class="computeroutput"><span class="keyword">int</span></code>.
Thus, dereferencing <code class="computeroutput"><span class="identifier">x</span></code> now
returns an <code class="computeroutput"><span class="keyword">int</span></code>.
</p>
<p>
<code class="computeroutput"><a class="link" href="../boost/type_erasure/same_type.html" title="Struct template same_type">same_type</a></code> can
also be used for two placeholders. This allows us to use a simple name instead
of writing out an associated type over and over.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span>
<span class="identifier">pointer</span><span class="special"><>,</span>
<span class="identifier">same_type</span><span class="special"><</span><span class="identifier">pointer</span><span class="special"><>::</span><span class="identifier">element_type</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">>,</span>
<span class="identifier">typeid_</span><span class="special"><</span><span class="identifier">_a</span><span class="special">>,</span>
<span class="identifier">copy_constructible</span><span class="special"><</span><span class="identifier">_a</span><span class="special">>,</span>
<span class="identifier">addable</span><span class="special"><</span><span class="identifier">_a</span><span class="special">>,</span>
<span class="identifier">ostreamable</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">></span>
<span class="special">></span>
<span class="special">></span> <span class="identifier">x</span><span class="special">(&</span><span class="identifier">i</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">(*</span><span class="identifier">x</span> <span class="special">+</span> <span class="special">*</span><span class="identifier">x</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 20</span>
</pre>
<p>
</p>
</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 © 2011-2013 Steven Watanabe<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></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="multi.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_typeerasure.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="any.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
|
