extending.html
15.4 KB
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
<!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=UTF-8">
<title>Extending return type deduction system</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="../lambda.html" title="Chapter 18. Boost.Lambda">
<link rel="prev" href="le_in_details.html" title="Lambda expressions in details">
<link rel="next" href="s07.html" title="Practical considerations">
</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="le_in_details.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../lambda.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="s07.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="lambda.extending"></a>Extending return type deduction system</h2></div></div></div>
<p>
In this section, we explain how to extend the return type deduction system
to cover user defined operators.
In many cases this is not necessary,
as the BLL defines default return types for operators.
For example, the default return type for all comparison operators is
<code class="literal">bool</code>, and as long as the user defined comparison operators
have a bool return type, there is no need to write new specializations
for the return type deduction classes.
Sometimes this cannot be avoided, though.
</p>
<p>
The overloadable user defined operators are either unary or binary.
For each arity, there are two traits templates that define the
return types of the different operators.
Hence, the return type system can be extended by providing more
specializations for these templates.
The templates for unary functors are
<code class="literal">
plain_return_type_1<Action, A>
</code>
and
<code class="literal">
return_type_1<Action, A>
</code>, and
<code class="literal">
plain_return_type_2<Action, A, B>
</code>
and
<code class="literal">
return_type_2<Action, A, B>
</code>
respectively for binary functors.
</p>
<p>
The first parameter (<code class="literal">Action</code>) to all these templates
is the <span class="emphasis"><em>action</em></span> class, which specifies the operator.
Operators with similar return type rules are grouped together into
<span class="emphasis"><em>action groups</em></span>,
and only the action class and action group together define the operator
unambiguously.
As an example, the action type
<code class="literal">arithmetic_action<plus_action></code> stands for
<code class="literal">operator+</code>.
The complete listing of different action types is shown in
<a class="xref" href="extending.html#table:actions" title="Table 18.2. Action types">Table 18.2, “Action types”</a>.
</p>
<p>
The latter parameters, <code class="literal">A</code> in the unary case,
or <code class="literal">A</code> and <code class="literal">B</code> in the binary case,
stand for the argument types of the operator call.
The two sets of templates,
<code class="literal">plain_return_type_<em class="parameter"><code>n</code></em></code> and
<code class="literal">return_type_<em class="parameter"><code>n</code></em></code>
(<em class="parameter"><code>n</code></em> is 1 or 2) differ in the way how parameter types
are presented to them.
For the former templates, the parameter types are always provided as
non-reference types, and do not have const or volatile qualifiers.
This makes specializing easy, as commonly one specialization for each
user defined operator, or operator group, is enough.
On the other hand, if a particular operator is overloaded for different
cv-qualifications of the same argument types,
and the return types of these overloaded versions differ, a more fine-grained control is needed.
Hence, for the latter templates, the parameter types preserve the
cv-qualifiers, and are non-reference types as well.
The downside is, that for an overloaded set of operators of the
kind described above, one may end up needing up to
16 <code class="literal">return_type_2</code> specializations.
</p>
<p>
Suppose the user has overloaded the following operators for some user defined
types <code class="literal">X</code>, <code class="literal">Y</code> and <code class="literal">Z</code>:
</p>
<pre class="programlisting">
Z operator+(const X&, const Y&);
Z operator-(const X&, const Y&);
</pre>
<p>
Now, one can add a specialization stating, that if the left hand argument
is of type <code class="literal">X</code>, and the right hand one of type
<code class="literal">Y</code>, the return type of all such binary arithmetic
operators is <code class="literal">Z</code>:
</p>
<pre class="programlisting">
namespace boost {
namespace lambda {
template<class Act>
struct plain_return_type_2<arithmetic_action<Act>, X, Y> {
typedef Z type;
};
}
}
</pre>
<p>
Having this specialization defined, BLL is capable of correctly
deducing the return type of the above two operators.
Note, that the specializations must be in the same namespace,
<code class="literal">::boost::lambda</code>, with the primary template.
For brevity, we do not show the namespace definitions in the examples below.
</p>
<p>
It is possible to specialize on the level of an individual operator as well,
in addition to providing a specialization for a group of operators.
Say, we add a new arithmetic operator for argument types <code class="literal">X</code>
and <code class="literal">Y</code>:
</p>
<pre class="programlisting">
X operator*(const X&, const Y&);
</pre>
<p>
Our first rule for all arithmetic operators specifies that the return
type of this operator is <code class="literal">Z</code>,
which obviously is not the case.
Hence, we provide a new rule for the multiplication operator:
</p>
<pre class="programlisting">
template<>
struct plain_return_type_2<arithmetic_action<multiply_action>, X, Y> {
typedef X type;
};
</pre>
<p>
</p>
<p>
The specializations can define arbitrary mappings from the argument types
to the return type.
Suppose we have some mathematical vector type, templated on the element type:
</p>
<pre class="programlisting">
template <class T> class my_vector;
</pre>
<p>
Suppose the addition operator is defined between any two
<code class="literal">my_vector</code> instantiations,
as long as the addition operator is defined between their element types.
Furthermore, the element type of the resulting <code class="literal">my_vector</code>
is the same as the result type of the addition between the element types.
E.g., adding <code class="literal">my_vector<int></code> and
<code class="literal">my_vector<double></code> results in
<code class="literal">my_vector<double></code>.
The BLL has traits classes to perform the implicit built-in and standard
type conversions between integral, floating point, and complex classes.
Using BLL tools, the addition operator described above can be defined as:
</p>
<pre class="programlisting">
template<class A, class B>
my_vector<typename return_type_2<arithmetic_action<plus_action>, A, B>::type>
operator+(const my_vector<A>& a, const my_vector<B>& b)
{
typedef typename
return_type_2<arithmetic_action<plus_action>, A, B>::type res_type;
return my_vector<res_type>();
}
</pre>
<p>
</p>
<p>
To allow BLL to deduce the type of <code class="literal">my_vector</code>
additions correctly, we can define:
</p>
<pre class="programlisting">
template<class A, class B>
class plain_return_type_2<arithmetic_action<plus_action>,
my_vector<A>, my_vector<B> > {
typedef typename
return_type_2<arithmetic_action<plus_action>, A, B>::type res_type;
public:
typedef my_vector<res_type> type;
};
</pre>
<p>
Note, that we are reusing the existing specializations for the
BLL <code class="literal">return_type_2</code> template,
which require that the argument types are references.
</p>
<div class="table">
<a name="table:actions"></a><p class="title"><b>Table 18.2. Action types</b></p>
<div class="table-contents"><table class="table" summary="Action types">
<colgroup>
<col>
<col>
</colgroup>
<tbody>
<tr>
<td><code class="literal">+</code></td>
<td><code class="literal">arithmetic_action<plus_action></code></td>
</tr>
<tr>
<td><code class="literal">-</code></td>
<td><code class="literal">arithmetic_action<minus_action></code></td>
</tr>
<tr>
<td><code class="literal">*</code></td>
<td><code class="literal">arithmetic_action<multiply_action></code></td>
</tr>
<tr>
<td><code class="literal">/</code></td>
<td><code class="literal">arithmetic_action<divide_action></code></td>
</tr>
<tr>
<td><code class="literal">%</code></td>
<td><code class="literal">arithmetic_action<remainder_action></code></td>
</tr>
<tr>
<td><code class="literal">+</code></td>
<td><code class="literal">unary_arithmetic_action<plus_action></code></td>
</tr>
<tr>
<td><code class="literal">-</code></td>
<td><code class="literal">unary_arithmetic_action<minus_action></code></td>
</tr>
<tr>
<td><code class="literal">&</code></td>
<td><code class="literal">bitwise_action<and_action></code></td>
</tr>
<tr>
<td><code class="literal">|</code></td>
<td><code class="literal">bitwise_action<or_action></code></td>
</tr>
<tr>
<td><code class="literal">~</code></td>
<td><code class="literal">bitwise_action<not_action></code></td>
</tr>
<tr>
<td><code class="literal">^</code></td>
<td><code class="literal">bitwise_action<xor_action></code></td>
</tr>
<tr>
<td><code class="literal"><<</code></td>
<td><code class="literal">bitwise_action<leftshift_action_no_stream></code></td>
</tr>
<tr>
<td><code class="literal">>></code></td>
<td><code class="literal">bitwise_action<rightshift_action_no_stream></code></td>
</tr>
<tr>
<td><code class="literal">&&</code></td>
<td><code class="literal">logical_action<and_action></code></td>
</tr>
<tr>
<td><code class="literal">||</code></td>
<td><code class="literal">logical_action<or_action></code></td>
</tr>
<tr>
<td><code class="literal">!</code></td>
<td><code class="literal">logical_action<not_action></code></td>
</tr>
<tr>
<td><code class="literal"><</code></td>
<td><code class="literal">relational_action<less_action></code></td>
</tr>
<tr>
<td><code class="literal">></code></td>
<td><code class="literal">relational_action<greater_action></code></td>
</tr>
<tr>
<td><code class="literal"><=</code></td>
<td><code class="literal">relational_action<lessorequal_action></code></td>
</tr>
<tr>
<td><code class="literal">>=</code></td>
<td><code class="literal">relational_action<greaterorequal_action></code></td>
</tr>
<tr>
<td><code class="literal">==</code></td>
<td><code class="literal">relational_action<equal_action></code></td>
</tr>
<tr>
<td><code class="literal">!=</code></td>
<td><code class="literal">relational_action<notequal_action></code></td>
</tr>
<tr>
<td><code class="literal">+=</code></td>
<td><code class="literal">arithmetic_assignment_action<plus_action></code></td>
</tr>
<tr>
<td><code class="literal">-=</code></td>
<td><code class="literal">arithmetic_assignment_action<minus_action></code></td>
</tr>
<tr>
<td><code class="literal">*=</code></td>
<td><code class="literal">arithmetic_assignment_action<multiply_action></code></td>
</tr>
<tr>
<td><code class="literal">/=</code></td>
<td><code class="literal">arithmetic_assignment_action<divide_action></code></td>
</tr>
<tr>
<td><code class="literal">%=</code></td>
<td><code class="literal">arithmetic_assignment_action<remainder_action></code></td>
</tr>
<tr>
<td><code class="literal">&=</code></td>
<td><code class="literal">bitwise_assignment_action<and_action></code></td>
</tr>
<tr>
<td><code class="literal">=|</code></td>
<td><code class="literal">bitwise_assignment_action<or_action></code></td>
</tr>
<tr>
<td><code class="literal">^=</code></td>
<td><code class="literal">bitwise_assignment_action<xor_action></code></td>
</tr>
<tr>
<td><code class="literal"><<=</code></td>
<td><code class="literal">bitwise_assignment_action<leftshift_action></code></td>
</tr>
<tr>
<td><code class="literal">>>=</code></td>
<td><code class="literal">bitwise_assignment_action<rightshift_action></code></td>
</tr>
<tr>
<td><code class="literal">++</code></td>
<td><code class="literal">pre_increment_decrement_action<increment_action></code></td>
</tr>
<tr>
<td><code class="literal">--</code></td>
<td><code class="literal">pre_increment_decrement_action<decrement_action></code></td>
</tr>
<tr>
<td><code class="literal">++</code></td>
<td><code class="literal">post_increment_decrement_action<increment_action></code></td>
</tr>
<tr>
<td><code class="literal">--</code></td>
<td><code class="literal">post_increment_decrement_action<decrement_action></code></td>
</tr>
<tr>
<td><code class="literal">&</code></td>
<td><code class="literal">other_action<address_of_action></code></td>
</tr>
<tr>
<td><code class="literal">*</code></td>
<td><code class="literal">other_action<contents_of_action></code></td>
</tr>
<tr>
<td><code class="literal">,</code></td>
<td><code class="literal">other_action<comma_action></code></td>
</tr>
<tr>
<td><code class="literal">->*</code></td>
<td><code class="literal">other_action<member_pointer_action></code></td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break">
</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 © 1999-2004 Jaakko Järvi, Gary Powell<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="le_in_details.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../lambda.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="s07.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>