]>
Commit | Line | Data |
---|---|---|
7c673cae FG |
1 | [/ |
2 | (C) Copyright Edward Diener 2011 | |
3 | Distributed under the Boost Software License, Version 1.0. | |
4 | (See accompanying file LICENSE_1_0.txt or copy at | |
5 | http://www.boost.org/LICENSE_1_0.txt). | |
6 | ] | |
7 | ||
8 | [section:tti_func_templates Introspecting Function Templates] | |
9 | ||
10 | The one nested element which the TTI library does not introspect is | |
11 | function templates. | |
12 | ||
13 | Function templates, like functions, can be member function templates or | |
14 | static member function templates. In this respect they are related to | |
15 | functions. Function templates represent a family of possible functions. | |
16 | In this respect they are similar to class templates, which represent a | |
17 | family of possible class types. | |
18 | ||
19 | The technique for introspecting class templates in the TTI library is taken | |
20 | from the implementation of the technique in the Boost MPL library. In the case | |
21 | of `BOOST_TTI_HAS_TEMPLATE` it directly uses the Boost MPL library functionality | |
22 | while in the case of `BOOST_TTI_HAS_TEMPLATE_CHECK_PARAMS` it replicates much | |
23 | of the technique in the Boost MPL library. The technique depends directly on | |
24 | the fact that in C++ we can pass a template as a parameter to another template | |
25 | using what is called a "template template" parameter type. | |
26 | ||
27 | One obvious thing about a template template parameter type is that it is a | |
28 | class template. For whatever historical or technical reasons, no one has ever | |
29 | proposed that C++ have a way of passing a function template directly as a template | |
30 | parameter, perhaps to be called a "function template template" parameter type. | |
31 | I personally think this would be a good addition to C++ and would | |
32 | make the ability of passing a template as a parameter to another template | |
33 | more orthogonal, since both class templates and function templates would be supported. | |
34 | My efforts to discuss this on the major C++ newsgroups have | |
35 | met with arguments both against its practical usage and the justification | |
36 | that one can pass a function template to another template nested in a non-template | |
37 | class, which serves as a type. But of course we can do the same thing with class templates, | |
38 | which is in fact what Boost MPL does to pass templates as metadata, yet we still have | |
39 | template template parameters as class templates. | |
40 | ||
41 | Nonetheless the fact that we can pass class templates as a template parameter but not | |
42 | function templates as a template parameter is the major factor why there is no really good | |
43 | method for introspecting function templates at compile time. | |
44 | ||
45 | [heading Instantiating a nested function template] | |
46 | ||
47 | There is, however, an alternate but less certain way of introspecting a function template. | |
48 | I will endeavor to explain why this way is not currently included in the TTI library, | |
49 | but first I will explain what it is. | |
50 | ||
51 | It is possible to check whether some particular [*instantiation] of a nested function | |
52 | template exists at compile-time without generating a compiler error. Although checking if | |
53 | some particular instantiation of a nested function template exists at compile-time does | |
54 | not prove that the nested function template itself does or does not exist, | |
55 | since the instantiation itself may be incorrect and fail even when the nested function | |
56 | template exists, it provides a partial, if flawed, means of checking. | |
57 | ||
58 | The code to do this for member function templates looks like this | |
59 | ( similar code also exists for static member function templates ): | |
60 | ||
61 | template | |
62 | < | |
63 | class C, | |
64 | class T | |
65 | > | |
66 | struct TestFunctionTemplate | |
67 | { | |
68 | typedef char Bad; | |
69 | struct Good { char x[2]; }; | |
70 | template<T> struct helper; | |
71 | template<class U> static Good check(helper<&U::template SomeFuncTemplateName<int,long,double> > *); | |
72 | template<class U> static Bad check(...); | |
73 | static const bool value=sizeof(check<C>(0))==sizeof(Good); | |
74 | }; | |
75 | ||
76 | where 'SomeFuncTemplateName' is the name of the nested function template, | |
77 | followed by some parameters to instantiate it. The 'class C' is the type of | |
78 | the enclosing class and the 'class T' is the type of the instantiated member | |
79 | function template as a member function. | |
80 | ||
81 | As an example if we had: | |
82 | ||
83 | struct AType | |
84 | { | |
85 | template<class X,class Y,class Z> double SomeFuncTemplateName(X,Y *,Z &) { return 0.0; } | |
86 | }; | |
87 | ||
88 | then instantiating the above template with: | |
89 | ||
90 | TestFunctionTemplate | |
91 | < | |
92 | AType, | |
93 | double (AType::*)(int,long *,double &) | |
94 | > | |
95 | ||
96 | would provide a compile-time boolean value which would tell us whether the | |
97 | nested member function template exists for the particular instantiation | |
98 | provided above. Furthermore, through the use of a macro, the TTI library | |
99 | could provide the means for specifying the name of the nested member function | |
100 | template ('SomeFuncTemplateName' above) and its set of instantiated | |
101 | parameters ('int,long,double' above) for generating the template. | |
102 | ||
103 | So why does not the TTI library not provide at least this much functionality for | |
104 | introspecting member function templates, even if it represents a partially flawed | |
105 | way of doing so ? | |
106 | ||
107 | The reason is stunningly disappointing. Although the above code is perfectly correct C++ | |
108 | code ( 'clang' works correctly ), two of the major C++ compilers, in all of their different | |
109 | releases, can not handle the above code correctly. Both gcc ( g++ ) and Visual C++ incorrectly | |
110 | choose the wrong 'check' function even when the correct 'check' function applies ( Comeau C++ | |
111 | also fails but I am less concerned about that compiler since it is not used nearly as much as | |
112 | the other two ). All my attempts at alternatives to the above code have also failed. The problems | |
113 | with both compilers, in this regard, can be seen more easily with this snippet: | |
114 | ||
115 | struct AType | |
116 | { | |
117 | template<class AA> void SomeFuncTemplate() { } | |
118 | }; | |
119 | ||
120 | template<class T> | |
121 | struct Test | |
122 | { | |
123 | template<T> struct helper; | |
124 | template<class U> static void check(helper<&U::template SomeFuncTemplate<int> > *) { } | |
125 | }; | |
126 | ||
127 | int main() | |
128 | { | |
129 | Test< void (AType::*)() >::check<AType>(0); | |
130 | return 0; | |
131 | } | |
132 | ||
133 | Both compilers report compile errors with this perfectly correct code, | |
134 | ||
135 | gcc: | |
136 | ||
137 | error: no matching function for call to 'Test<void (AType::*)()>::check(int)' | |
138 | ||
139 | and msvc: | |
140 | ||
141 | error C2770: invalid explicit template argument(s) for 'void Test<T>::check(Test<T>::helper<&U::SomeFuncTemplate<int>> *)' | |
142 | ||
143 | There is a workaround for these compiler problems, which is to hardcode the name | |
144 | of the enclosing class, via a macro, in the generated template rather than pass it as a | |
145 | template type. In that case both compilers can handle both the member function code and | |
146 | the code snippet above correctly. In essence, when the line: | |
147 | ||
148 | template<class U> static void check(helper<&U::template SomeFuncTemplate<int> > *) { } | |
149 | ||
150 | gets replaced by: | |
151 | ||
152 | template<class U> static void check(helper<&AType::template SomeFuncTemplate<int> > *) { } | |
153 | ||
154 | both gcc and Visual C++ work correctly. The same goes for the 'check' line in the | |
155 | 'TestFunctionTemplate' above. | |
156 | ||
157 | But the workaround destroys one of the basic tenets of the TTI library, which is that | |
158 | the enclosing class be passed as a template parameter, especially as the enclosing class | |
159 | need not actually exist ( see `BOOST_TTI_MEMBER_TYPE` and the previous discussion of 'Nested Types' ), | |
160 | without producing a compiler error. So I have decided not to implement even this methodology to | |
161 | introspect nested function templates in the TTI library. | |
162 | ||
163 | [endsect] |