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9 | <title>Call Traits</title> | |
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12 | <body bgcolor="#FFFFFF" text="#000000" link="#0000FF" | |
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14 | ||
15 | <h1><img src="../../boost.png" width="276" height="86">Header | |
16 | <<a href="../../boost/detail/call_traits.hpp">boost/call_traits.hpp</a>></h1> | |
17 | ||
18 | <p>All of the contents of <boost/call_traits.hpp> are | |
19 | defined inside namespace boost.</p> | |
20 | ||
21 | <p>The template class call_traits<T> encapsulates the | |
22 | "best" method to pass a parameter of some type T to or | |
23 | from a function, and consists of a collection of typedefs defined | |
24 | as in the table below. The purpose of call_traits is to ensure | |
25 | that problems like "<a href="#refs">references to references</a>" | |
26 | never occur, and that parameters are passed in the most efficient | |
27 | manner possible (see <a href="#examples">examples</a>). In each | |
28 | case if your existing practice is to use the type defined on the | |
29 | left, then replace it with the call_traits defined type on the | |
30 | right. </p> | |
31 | ||
32 | <p>Note that for compilers that do not support either partial | |
33 | specialization or member templates, no benefit will occur from | |
34 | using call_traits: the call_traits defined types will always be | |
35 | the same as the existing practice in this case. In addition if | |
36 | only member templates and not partial template specialisation is | |
37 | support by the compiler (for example Visual C++ 6) then | |
38 | call_traits can not be used with array types (although it can be | |
39 | used to solve the reference to reference problem).</p> | |
40 | ||
41 | <table border="0" cellpadding="7" cellspacing="1" width="797"> | |
42 | <tr> | |
43 | <td valign="top" width="17%" bgcolor="#008080"><p | |
44 | align="center">Existing practice</p> | |
45 | </td> | |
46 | <td valign="top" width="35%" bgcolor="#008080"><p | |
47 | align="center">call_traits equivalent</p> | |
48 | </td> | |
49 | <td valign="top" width="32%" bgcolor="#008080"><p | |
50 | align="center">Description</p> | |
51 | </td> | |
52 | <td valign="top" width="16%" bgcolor="#008080"><p | |
53 | align="center">Notes</p> | |
54 | </td> | |
55 | </tr> | |
56 | <tr> | |
57 | <td valign="top" width="17%"><p align="center">T<br> | |
58 | (return by value)</p> | |
59 | </td> | |
60 | <td valign="top" width="35%"><p align="center"><code>call_traits<T>::value_type</code></p> | |
61 | </td> | |
62 | <td valign="top" width="32%">Defines a type that | |
63 | represents the "value" of type T. Use this for | |
64 | functions that return by value, or possibly for stored | |
65 | values of type T.</td> | |
66 | <td valign="top" width="16%"><p align="center">2</p> | |
67 | </td> | |
68 | </tr> | |
69 | <tr> | |
70 | <td valign="top" width="17%"><p align="center">T&<br> | |
71 | (return value)</p> | |
72 | </td> | |
73 | <td valign="top" width="35%"><p align="center"><code>call_traits<T>::reference</code></p> | |
74 | </td> | |
75 | <td valign="top" width="32%">Defines a type that | |
76 | represents a reference to type T. Use for functions that | |
77 | would normally return a T&.</td> | |
78 | <td valign="top" width="16%"><p align="center">1</p> | |
79 | </td> | |
80 | </tr> | |
81 | <tr> | |
82 | <td valign="top" width="17%"><p align="center">const | |
83 | T&<br> | |
84 | (return value)</p> | |
85 | </td> | |
86 | <td valign="top" width="35%"><p align="center"><code>call_traits<T>::const_reference</code></p> | |
87 | </td> | |
88 | <td valign="top" width="32%">Defines a type that | |
89 | represents a constant reference to type T. Use for | |
90 | functions that would normally return a const T&.</td> | |
91 | <td valign="top" width="16%"><p align="center">1</p> | |
92 | </td> | |
93 | </tr> | |
94 | <tr> | |
95 | <td valign="top" width="17%"><p align="center">const | |
96 | T&<br> | |
97 | (function parameter)</p> | |
98 | </td> | |
99 | <td valign="top" width="35%"><p align="center"><code>call_traits<T>::param_type</code></p> | |
100 | </td> | |
101 | <td valign="top" width="32%">Defines a type that | |
102 | represents the "best" way to pass a parameter | |
103 | of type T to a function.</td> | |
104 | <td valign="top" width="16%"><p align="center">1,3</p> | |
105 | </td> | |
106 | </tr> | |
107 | </table> | |
108 | ||
109 | <p>Notes:</p> | |
110 | ||
111 | <ol> | |
112 | <li>If T is already reference type, then call_traits is | |
113 | defined such that <a href="#refs">references to | |
114 | references</a> do not occur (requires partial | |
115 | specialization).</li> | |
116 | <li>If T is an array type, then call_traits defines <code>value_type</code> | |
117 | as a "constant pointer to type" rather than an | |
118 | "array of type" (requires partial | |
119 | specialization). Note that if you are using value_type as | |
120 | a stored value then this will result in storing a "constant | |
121 | pointer to an array" rather than the array itself. | |
122 | This may or may not be a good thing depending upon what | |
123 | you actually need (in other words take care!).</li> | |
124 | <li>If T is a small built in type or a pointer, then <code>param_type</code> | |
125 | is defined as <code>T const</code>, instead of <code>T | |
126 | const&</code>. This can improve the ability of the | |
127 | compiler to optimize loops in the body of the function if | |
128 | they depend upon the passed parameter, the semantics of | |
129 | the passed parameter is otherwise unchanged (requires | |
130 | partial specialization).</li> | |
131 | </ol> | |
132 | ||
133 | <p> </p> | |
134 | ||
135 | <h3>Copy constructibility</h3> | |
136 | ||
137 | <p>The following table defines which call_traits types can always | |
138 | be copy-constructed from which other types, those entries marked | |
139 | with a '?' are true only if and only if T is copy constructible:</p> | |
140 | ||
141 | <table border="0" cellpadding="7" cellspacing="1" width="766"> | |
142 | <tr> | |
143 | <td valign="top" width="17%"> </td> | |
144 | <td valign="top" colspan="5" width="85%" | |
145 | bgcolor="#008080"><p align="center">To:</p> | |
146 | </td> | |
147 | </tr> | |
148 | <tr> | |
149 | <td valign="top" width="17%" bgcolor="#008080">From:</td> | |
150 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
151 | align="center">T</p> | |
152 | </td> | |
153 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
154 | align="center">value_type</p> | |
155 | </td> | |
156 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
157 | align="center">reference</p> | |
158 | </td> | |
159 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
160 | align="center">const_reference</p> | |
161 | </td> | |
162 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
163 | align="center">param_type</p> | |
164 | </td> | |
165 | </tr> | |
166 | <tr> | |
167 | <td valign="top" width="17%" bgcolor="#C0C0C0">T</td> | |
168 | <td valign="top" width="17%"><p align="center">?</p> | |
169 | </td> | |
170 | <td valign="top" width="17%"><p align="center">?</p> | |
171 | </td> | |
172 | <td valign="top" width="17%"><p align="center">Y</p> | |
173 | </td> | |
174 | <td valign="top" width="17%"><p align="center">Y</p> | |
175 | </td> | |
176 | <td valign="top" width="17%"><p align="center">Y</p> | |
177 | </td> | |
178 | </tr> | |
179 | <tr> | |
180 | <td valign="top" width="17%" bgcolor="#C0C0C0">value_type</td> | |
181 | <td valign="top" width="17%"><p align="center">?</p> | |
182 | </td> | |
183 | <td valign="top" width="17%"><p align="center">?</p> | |
184 | </td> | |
185 | <td valign="top" width="17%"><p align="center">N</p> | |
186 | </td> | |
187 | <td valign="top" width="17%"><p align="center">N</p> | |
188 | </td> | |
189 | <td valign="top" width="17%"><p align="center">Y</p> | |
190 | </td> | |
191 | </tr> | |
192 | <tr> | |
193 | <td valign="top" width="17%" bgcolor="#C0C0C0">reference</td> | |
194 | <td valign="top" width="17%"><p align="center">?</p> | |
195 | </td> | |
196 | <td valign="top" width="17%"><p align="center">?</p> | |
197 | </td> | |
198 | <td valign="top" width="17%"><p align="center">Y</p> | |
199 | </td> | |
200 | <td valign="top" width="17%"><p align="center">Y</p> | |
201 | </td> | |
202 | <td valign="top" width="17%"><p align="center">Y</p> | |
203 | </td> | |
204 | </tr> | |
205 | <tr> | |
206 | <td valign="top" width="17%" bgcolor="#C0C0C0">const_reference</td> | |
207 | <td valign="top" width="17%"><p align="center">?</p> | |
208 | </td> | |
209 | <td valign="top" width="17%"><p align="center">N</p> | |
210 | </td> | |
211 | <td valign="top" width="17%"><p align="center">N</p> | |
212 | </td> | |
213 | <td valign="top" width="17%"><p align="center">Y</p> | |
214 | </td> | |
215 | <td valign="top" width="17%"><p align="center">Y</p> | |
216 | </td> | |
217 | </tr> | |
218 | <tr> | |
219 | <td valign="top" width="17%" bgcolor="#C0C0C0">param_type</td> | |
220 | <td valign="top" width="17%"><p align="center">?</p> | |
221 | </td> | |
222 | <td valign="top" width="17%"><p align="center">?</p> | |
223 | </td> | |
224 | <td valign="top" width="17%"><p align="center">N</p> | |
225 | </td> | |
226 | <td valign="top" width="17%"><p align="center">N</p> | |
227 | </td> | |
228 | <td valign="top" width="17%"><p align="center">Y</p> | |
229 | </td> | |
230 | </tr> | |
231 | </table> | |
232 | ||
233 | <p> </p> | |
234 | ||
235 | <p>If T is an assignable type the following assignments are | |
236 | possible:</p> | |
237 | ||
238 | <table border="0" cellpadding="7" cellspacing="1" width="766"> | |
239 | <tr> | |
240 | <td valign="top" width="17%"> </td> | |
241 | <td valign="top" colspan="5" width="85%" | |
242 | bgcolor="#008080"><p align="center">To:</p> | |
243 | </td> | |
244 | </tr> | |
245 | <tr> | |
246 | <td valign="top" width="17%" bgcolor="#008080">From:</td> | |
247 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
248 | align="center">T</p> | |
249 | </td> | |
250 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
251 | align="center">value_type</p> | |
252 | </td> | |
253 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
254 | align="center">reference</p> | |
255 | </td> | |
256 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
257 | align="center">const_reference</p> | |
258 | </td> | |
259 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
260 | align="center">param_type</p> | |
261 | </td> | |
262 | </tr> | |
263 | <tr> | |
264 | <td valign="top" width="17%" bgcolor="#C0C0C0">T</td> | |
265 | <td valign="top" width="17%"><p align="center">Y</p> | |
266 | </td> | |
267 | <td valign="top" width="17%"><p align="center">Y</p> | |
268 | </td> | |
269 | <td valign="top" width="17%"><p align="center">-</p> | |
270 | </td> | |
271 | <td valign="top" width="17%"><p align="center">-</p> | |
272 | </td> | |
273 | <td valign="top" width="17%"><p align="center">-</p> | |
274 | </td> | |
275 | </tr> | |
276 | <tr> | |
277 | <td valign="top" width="17%" bgcolor="#C0C0C0">value_type</td> | |
278 | <td valign="top" width="17%"><p align="center">Y</p> | |
279 | </td> | |
280 | <td valign="top" width="17%"><p align="center">Y</p> | |
281 | </td> | |
282 | <td valign="top" width="17%"><p align="center">-</p> | |
283 | </td> | |
284 | <td valign="top" width="17%"><p align="center">-</p> | |
285 | </td> | |
286 | <td valign="top" width="17%"><p align="center">-</p> | |
287 | </td> | |
288 | </tr> | |
289 | <tr> | |
290 | <td valign="top" width="17%" bgcolor="#C0C0C0">reference</td> | |
291 | <td valign="top" width="17%"><p align="center">Y</p> | |
292 | </td> | |
293 | <td valign="top" width="17%"><p align="center">Y</p> | |
294 | </td> | |
295 | <td valign="top" width="17%"><p align="center">-</p> | |
296 | </td> | |
297 | <td valign="top" width="17%"><p align="center">-</p> | |
298 | </td> | |
299 | <td valign="top" width="17%"><p align="center">-</p> | |
300 | </td> | |
301 | </tr> | |
302 | <tr> | |
303 | <td valign="top" width="17%" bgcolor="#C0C0C0">const_reference</td> | |
304 | <td valign="top" width="17%"><p align="center">Y</p> | |
305 | </td> | |
306 | <td valign="top" width="17%"><p align="center">Y</p> | |
307 | </td> | |
308 | <td valign="top" width="17%"><p align="center">-</p> | |
309 | </td> | |
310 | <td valign="top" width="17%"><p align="center">-</p> | |
311 | </td> | |
312 | <td valign="top" width="17%"><p align="center">-</p> | |
313 | </td> | |
314 | </tr> | |
315 | <tr> | |
316 | <td valign="top" width="17%" bgcolor="#C0C0C0">param_type</td> | |
317 | <td valign="top" width="17%"><p align="center">Y</p> | |
318 | </td> | |
319 | <td valign="top" width="17%"><p align="center">Y</p> | |
320 | </td> | |
321 | <td valign="top" width="17%"><p align="center">-</p> | |
322 | </td> | |
323 | <td valign="top" width="17%"><p align="center">-</p> | |
324 | </td> | |
325 | <td valign="top" width="17%"><p align="center">-</p> | |
326 | </td> | |
327 | </tr> | |
328 | </table> | |
329 | ||
330 | <p> </p> | |
331 | ||
332 | <h3><a name="examples"></a>Examples</h3> | |
333 | ||
334 | <p>The following table shows the effect that call_traits has on | |
335 | various types, the table assumes that the compiler supports | |
336 | partial specialization: if it doesn't then all types behave in | |
337 | the same way as the entry for "myclass", and | |
338 | call_traits can not be used with reference or array types.</p> | |
339 | ||
340 | <table border="0" cellpadding="7" cellspacing="1" width="766"> | |
341 | <tr> | |
342 | <td valign="top" width="17%"> </td> | |
343 | <td valign="top" colspan="5" width="85%" | |
344 | bgcolor="#008080"><p align="center">Call_traits type:</p> | |
345 | </td> | |
346 | </tr> | |
347 | <tr> | |
348 | <td valign="top" width="17%" bgcolor="#008080"><p | |
349 | align="center">Original type T</p> | |
350 | </td> | |
351 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
352 | align="center">value_type</p> | |
353 | </td> | |
354 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
355 | align="center">reference</p> | |
356 | </td> | |
357 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
358 | align="center">const_reference</p> | |
359 | </td> | |
360 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
361 | align="center">param_type</p> | |
362 | </td> | |
363 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
364 | align="center">Applies to:</p> | |
365 | </td> | |
366 | </tr> | |
367 | <tr> | |
368 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
369 | align="center">myclass</p> | |
370 | </td> | |
371 | <td valign="top" width="17%"><p align="center">myclass</p> | |
372 | </td> | |
373 | <td valign="top" width="17%"><p align="center">myclass&</p> | |
374 | </td> | |
375 | <td valign="top" width="17%"><p align="center">const | |
376 | myclass&</p> | |
377 | </td> | |
378 | <td valign="top" width="17%"><p align="center">myclass | |
379 | const&</p> | |
380 | </td> | |
381 | <td valign="top" width="17%"><p align="center">All user | |
382 | defined types.</p> | |
383 | </td> | |
384 | </tr> | |
385 | <tr> | |
386 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
387 | align="center">int</p> | |
388 | </td> | |
389 | <td valign="top" width="17%"><p align="center">int</p> | |
390 | </td> | |
391 | <td valign="top" width="17%"><p align="center">int&</p> | |
392 | </td> | |
393 | <td valign="top" width="17%"><p align="center">const | |
394 | int&</p> | |
395 | </td> | |
396 | <td valign="top" width="17%"><p align="center">int const</p> | |
397 | </td> | |
398 | <td valign="top" width="17%"><p align="center">All small | |
399 | built-in types.</p> | |
400 | </td> | |
401 | </tr> | |
402 | <tr> | |
403 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
404 | align="center">int*</p> | |
405 | </td> | |
406 | <td valign="top" width="17%"><p align="center">int*</p> | |
407 | </td> | |
408 | <td valign="top" width="17%"><p align="center">int*&</p> | |
409 | </td> | |
410 | <td valign="top" width="17%"><p align="center">int*const&</p> | |
411 | </td> | |
412 | <td valign="top" width="17%"><p align="center">int* const</p> | |
413 | </td> | |
414 | <td valign="top" width="17%"><p align="center">All | |
415 | pointer types.</p> | |
416 | </td> | |
417 | </tr> | |
418 | <tr> | |
419 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
420 | align="center">int&</p> | |
421 | </td> | |
422 | <td valign="top" width="17%"><p align="center">int&</p> | |
423 | </td> | |
424 | <td valign="top" width="17%"><p align="center">int&</p> | |
425 | </td> | |
426 | <td valign="top" width="17%"><p align="center">const | |
427 | int&</p> | |
428 | </td> | |
429 | <td valign="top" width="17%"><p align="center">int&</p> | |
430 | </td> | |
431 | <td valign="top" width="17%"><p align="center">All | |
432 | reference types.</p> | |
433 | </td> | |
434 | </tr> | |
435 | <tr> | |
436 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
437 | align="center">const int&</p> | |
438 | </td> | |
439 | <td valign="top" width="17%"><p align="center">const | |
440 | int&</p> | |
441 | </td> | |
442 | <td valign="top" width="17%"><p align="center">const | |
443 | int&</p> | |
444 | </td> | |
445 | <td valign="top" width="17%"><p align="center">const | |
446 | int&</p> | |
447 | </td> | |
448 | <td valign="top" width="17%"><p align="center">const | |
449 | int&</p> | |
450 | </td> | |
451 | <td valign="top" width="17%"><p align="center">All | |
452 | constant-references.</p> | |
453 | </td> | |
454 | </tr> | |
455 | <tr> | |
456 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
457 | align="center">int[3]</p> | |
458 | </td> | |
459 | <td valign="top" width="17%"><p align="center">const int*</p> | |
460 | </td> | |
461 | <td valign="top" width="17%"><p align="center">int(&)[3]</p> | |
462 | </td> | |
463 | <td valign="top" width="17%"><p align="center">const int(&)[3]</p> | |
464 | </td> | |
465 | <td valign="top" width="17%"><p align="center">const int* | |
466 | const</p> | |
467 | </td> | |
468 | <td valign="top" width="17%"><p align="center">All array | |
469 | types.</p> | |
470 | </td> | |
471 | </tr> | |
472 | <tr> | |
473 | <td valign="top" width="17%" bgcolor="#C0C0C0"><p | |
474 | align="center">const int[3]</p> | |
475 | </td> | |
476 | <td valign="top" width="17%"><p align="center">const int*</p> | |
477 | </td> | |
478 | <td valign="top" width="17%"><p align="center">const int(&)[3]</p> | |
479 | </td> | |
480 | <td valign="top" width="17%"><p align="center">const int(&)[3]</p> | |
481 | </td> | |
482 | <td valign="top" width="17%"><p align="center">const int* | |
483 | const</p> | |
484 | </td> | |
485 | <td valign="top" width="17%"><p align="center">All | |
486 | constant-array types.</p> | |
487 | </td> | |
488 | </tr> | |
489 | </table> | |
490 | ||
491 | <p> </p> | |
492 | ||
493 | <h4>Example 1:</h4> | |
494 | ||
495 | <p>The following class is a trivial class that stores some type T | |
92f5a8d4 | 496 | by value (see the <a href="test/call_traits_test.cpp">call_traits_test.cpp</a> |
7c673cae FG |
497 | file), the aim is to illustrate how each of the available |
498 | call_traits typedefs may be used:</p> | |
499 | ||
500 | <pre>template <class T> | |
501 | struct contained | |
502 | { | |
503 | // define our typedefs first, arrays are stored by value | |
504 | // so value_type is not the same as result_type: | |
505 | typedef typename boost::call_traits<T>::param_type param_type; | |
506 | typedef typename boost::call_traits<T>::reference reference; | |
507 | typedef typename boost::call_traits<T>::const_reference const_reference; | |
508 | typedef T value_type; | |
509 | typedef typename boost::call_traits<T>::value_type result_type; | |
510 | ||
511 | // stored value: | |
512 | value_type v_; | |
513 | ||
514 | // constructors: | |
515 | contained() {} | |
516 | contained(param_type p) : v_(p){} | |
517 | // return byval: | |
518 | result_type value() { return v_; } | |
519 | // return by_ref: | |
520 | reference get() { return v_; } | |
521 | const_reference const_get()const { return v_; } | |
522 | // pass value: | |
523 | void call(param_type p){} | |
524 | ||
525 | };</pre> | |
526 | ||
527 | <h4><a name="refs"></a>Example 2 (the reference to reference | |
528 | problem):</h4> | |
529 | ||
530 | <p>Consider the definition of std::binder1st:</p> | |
531 | ||
532 | <pre>template <class Operation> | |
533 | class binder1st : | |
534 | public unary_function<typename Operation::second_argument_type, typename Operation::result_type> | |
535 | { | |
536 | protected: | |
537 | Operation op; | |
538 | typename Operation::first_argument_type value; | |
539 | public: | |
540 | binder1st(const Operation& x, const typename Operation::first_argument_type& y); | |
541 | typename Operation::result_type operator()(const typename Operation::second_argument_type& x) const; | |
542 | }; </pre> | |
543 | ||
544 | <p>Now consider what happens in the relatively common case that | |
545 | the functor takes its second argument as a reference, that | |
546 | implies that <code>Operation::second_argument_type</code> is a | |
547 | reference type, <code>operator()</code> will now end up taking a | |
548 | reference to a reference as an argument, and that is not | |
549 | currently legal. The solution here is to modify <code>operator()</code> | |
550 | to use call_traits:</p> | |
551 | ||
552 | <pre>typename Operation::result_type operator()(typename call_traits<typename Operation::second_argument_type>::param_type x) const;</pre> | |
553 | ||
554 | <p>Now in the case that <code>Operation::second_argument_type</code> | |
555 | is a reference type, the argument is passed as a reference, and | |
556 | the no "reference to reference" occurs.</p> | |
557 | ||
558 | <h4><a name="ex3"></a>Example 3 (the make_pair problem):</h4> | |
559 | ||
560 | <p>If we pass the name of an array as one (or both) arguments to <code>std::make_pair</code>, | |
561 | then template argument deduction deduces the passed parameter as | |
562 | "const reference to array of T", this also applies to | |
563 | string literals (which are really array literals). Consequently | |
564 | instead of returning a pair of pointers, it tries to return a | |
565 | pair of arrays, and since an array type is not copy-constructible | |
566 | the code fails to compile. One solution is to explicitly cast the | |
567 | arguments to make_pair to pointers, but call_traits provides a | |
568 | better (i.e. automatic) solution (and one that works safely even | |
569 | in generic code where the cast might do the wrong thing):</p> | |
570 | ||
571 | <pre>template <class T1, class T2> | |
572 | std::pair< | |
573 | typename boost::call_traits<T1>::value_type, | |
574 | typename boost::call_traits<T2>::value_type> | |
575 | make_pair(const T1& t1, const T2& t2) | |
576 | { | |
577 | return std::pair< | |
578 | typename boost::call_traits<T1>::value_type, | |
579 | typename boost::call_traits<T2>::value_type>(t1, t2); | |
580 | }</pre> | |
581 | ||
582 | <p>Here, the deduced argument types will be automatically | |
583 | degraded to pointers if the deduced types are arrays, similar | |
584 | situations occur in the standard binders and adapters: in | |
585 | principle in any function that "wraps" a temporary | |
586 | whose type is deduced. Note that the function arguments to | |
587 | make_pair are not expressed in terms of call_traits: doing so | |
588 | would prevent template argument deduction from functioning.</p> | |
589 | ||
590 | <h4><a name="ex4"></a>Example 4 (optimising fill):</h4> | |
591 | ||
592 | <p>The call_traits template will "optimize" the passing | |
593 | of a small built-in type as a function parameter, this mainly has | |
594 | an effect when the parameter is used within a loop body. In the | |
595 | following example (see <a | |
596 | href="../type_traits/examples/fill_example.cpp">fill_example.cpp</a>), | |
597 | a version of std::fill is optimized in two ways: if the type | |
598 | passed is a single byte built-in type then std::memset is used to | |
599 | effect the fill, otherwise a conventional C++ implemention is | |
600 | used, but with the passed parameter "optimized" using | |
601 | call_traits:</p> | |
602 | ||
603 | <pre>namespace detail{ | |
604 | ||
605 | template <bool opt> | |
606 | struct filler | |
607 | { | |
608 | template <typename I, typename T> | |
609 | static void do_fill(I first, I last, typename boost::call_traits<T>::param_type val) | |
610 | { | |
611 | while(first != last) | |
612 | { | |
613 | *first = val; | |
614 | ++first; | |
615 | } | |
616 | } | |
617 | }; | |
618 | ||
619 | template <> | |
620 | struct filler<true> | |
621 | { | |
622 | template <typename I, typename T> | |
623 | static void do_fill(I first, I last, T val) | |
624 | { | |
625 | memset(first, val, last-first); | |
626 | } | |
627 | }; | |
628 | ||
629 | } | |
630 | ||
631 | template <class I, class T> | |
632 | inline void fill(I first, I last, const T& val) | |
633 | { | |
634 | enum{ can_opt = boost::is_pointer<I>::value | |
635 | && boost::is_arithmetic<T>::value | |
636 | && (sizeof(T) == 1) }; | |
637 | typedef detail::filler<can_opt> filler_t; | |
638 | filler_t::template do_fill<I,T>(first, last, val); | |
639 | }</pre> | |
640 | ||
641 | <p>Footnote: the reason that this is "optimal" for | |
642 | small built-in types is that with the value passed as "T | |
643 | const" instead of "const T&" the compiler is | |
644 | able to tell both that the value is constant and that it is free | |
645 | of aliases. With this information the compiler is able to cache | |
646 | the passed value in a register, unroll the loop, or use | |
647 | explicitly parallel instructions: if any of these are supported. | |
648 | Exactly how much mileage you will get from this depends upon your | |
649 | compiler - we could really use some accurate benchmarking | |
650 | software as part of boost for cases like this.</p> | |
651 | ||
652 | <p>Note that the function arguments to fill are not expressed in | |
653 | terms of call_traits: doing so would prevent template argument | |
654 | deduction from functioning. Instead fill acts as a "thin | |
655 | wrapper" that is there to perform template argument | |
656 | deduction, the compiler will optimise away the call to fill all | |
657 | together, replacing it with the call to filler<>::do_fill, | |
658 | which does use call_traits.</p> | |
659 | ||
660 | <h3>Rationale</h3> | |
661 | ||
662 | <p>The following notes are intended to briefly describe the | |
663 | rational behind choices made in call_traits.</p> | |
664 | ||
665 | <p>All user-defined types follow "existing practice" | |
666 | and need no comment.</p> | |
667 | ||
668 | <p>Small built-in types (what the standard calls fundamental | |
669 | types [3.9.1]) differ from existing practice only in the <i>param_type</i> | |
670 | typedef. In this case passing "T const" is compatible | |
671 | with existing practice, but may improve performance in some cases | |
672 | (see <a href="#ex4">Example 4</a>), in any case this should never | |
673 | be any worse than existing practice.</p> | |
674 | ||
675 | <p>Pointers follow the same rational as small built-in types.</p> | |
676 | ||
677 | <p>For reference types the rational follows <a href="#refs">Example | |
678 | 2</a> - references to references are not allowed, so the | |
679 | call_traits members must be defined such that these problems do | |
680 | not occur. There is a proposal to modify the language such that | |
681 | "a reference to a reference is a reference" (issue #106, | |
682 | submitted by Bjarne Stroustrup), call_traits<T>::value_type | |
683 | and call_traits<T>::param_type both provide the same effect | |
684 | as that proposal, without the need for a language change (in | |
685 | other words it's a workaround).</p> | |
686 | ||
687 | <p>For array types, a function that takes an array as an argument | |
688 | will degrade the array type to a pointer type: this means that | |
689 | the type of the actual parameter is different from its declared | |
690 | type, something that can cause endless problems in template code | |
691 | that relies on the declared type of a parameter. For example:</p> | |
692 | ||
693 | <pre>template <class T> | |
694 | struct A | |
695 | { | |
696 | void foo(T t); | |
697 | };</pre> | |
698 | ||
699 | <p><font face="Times New Roman">In this case if we instantiate | |
700 | A<int[2]> then the declared type of the parameter passed to | |
701 | member function foo is int[2], but it's actual type is const int*, | |
702 | if we try to use the type T within the function body, then there | |
703 | is a strong likelyhood that our code will not compile:</font></p> | |
704 | ||
705 | <pre>template <class T> | |
706 | void A<T>::foo(T t) | |
707 | { | |
708 | T dup(t); // doesn't compile for case that T is an array. | |
709 | }</pre> | |
710 | ||
711 | <p>By using call_traits the degradation from array to pointer is | |
712 | explicit, and the type of the parameter is the same as it's | |
713 | declared type:</p> | |
714 | ||
715 | <pre>template <class T> | |
716 | struct A | |
717 | { | |
718 | void foo(typename call_traits<T>::value_type t); | |
719 | }; | |
720 | ||
721 | template <class T> | |
722 | void A<T>::foo(typename call_traits<T>::value_type t) | |
723 | { | |
724 | typename call_traits<T>::value_type dup(t); // OK even if T is an array type. | |
725 | }</pre> | |
726 | ||
727 | <p>For value_type (return by value), again only a pointer may be | |
728 | returned, not a copy of the whole array, and again call_traits | |
729 | makes the degradation explicit. The value_type member is useful | |
730 | whenever an array must be explicitly degraded to a pointer - <a | |
731 | href="#ex3">Example 3</a> provides the test case (Footnote: the | |
732 | array specialisation for call_traits is the least well understood | |
733 | of all the call_traits specialisations, if the given semantics | |
734 | cause specific problems for you, or don't solve a particular | |
735 | array-related problem, then I would be interested to hear about | |
736 | it. Most people though will probably never need to use this | |
737 | specialisation).</p> | |
738 | ||
739 | <hr> | |
740 | ||
741 | <p>Revised 01 September 2000</p> | |
742 | ||
743 | <p> | |
744 | Copyright 2000 Steve Cleary, Beman Dawes, Howard | |
745 | Hinnant and John Maddock. <br/> | |
746 | Use, modification and distribution are subject to the | |
747 | Boost Software License, Version 1.0. | |
748 | (See accompanying file LICENSE_1_0.txt | |
749 | or copy at <a href="http://www.boost.org/LICENSE_1_0.txt"> | |
750 | http://www.boost.org/LICENSE_1_0.txt | |
751 | </a>). | |
752 | </p> | |
753 | </body> | |
754 | </html> | |
755 |