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- [/
- (C) Copyright Edward Diener 2011
- Distributed under the Boost Software License, Version 1.0.
- (See accompanying file LICENSE_1_0.txt or copy at
- http://www.boost.org/LICENSE_1_0.txt).
- ]
- [section:tti_func_templates Introspecting Function Templates]
- The one nested element which the TTI library does not introspect is
- function templates.
- Function templates, like functions, can be member function templates or
- static member function templates. In this respect they are related to
- functions. Function templates represent a family of possible functions.
- In this respect they are similar to class templates, which represent a
- family of possible class types.
- The technique for introspecting class templates in the TTI library is taken
- from the implementation of the technique in the Boost MPL library. In the case
- of `BOOST_TTI_HAS_TEMPLATE` it directly uses the Boost MPL library functionality
- while in the case of `BOOST_TTI_HAS_TEMPLATE_CHECK_PARAMS` it replicates much
- of the technique in the Boost MPL library. The technique depends directly on
- the fact that in C++ we can pass a template as a parameter to another template
- using what is called a "template template" parameter type.
- One obvious thing about a template template parameter type is that it is a
- class template. For whatever historical or technical reasons, no one has ever
- proposed that C++ have a way of passing a function template directly as a template
- parameter, perhaps to be called a "function template template" parameter type.
- I personally think this would be a good addition to C++ and would
- make the ability of passing a template as a parameter to another template
- more orthogonal, since both class templates and function templates would be supported.
- My efforts to discuss this on the major C++ newsgroups have
- met with arguments both against its practical usage and the justification
- that one can pass a function template to another template nested in a non-template
- class, which serves as a type. But of course we can do the same thing with class templates,
- which is in fact what Boost MPL does to pass templates as metadata, yet we still have
- template template parameters as class templates.
- Nonetheless the fact that we can pass class templates as a template parameter but not
- function templates as a template parameter is the major factor why there is no really good
- method for introspecting function templates at compile time.
- [heading Instantiating a nested function template]
- There is, however, an alternate but less certain way of introspecting a function template.
- I will endeavor to explain why this way is not currently included in the TTI library,
- but first I will explain what it is.
- It is possible to check whether some particular [*instantiation] of a nested function
- template exists at compile-time without generating a compiler error. Although checking if
- some particular instantiation of a nested function template exists at compile-time does
- not prove that the nested function template itself does or does not exist,
- since the instantiation itself may be incorrect and fail even when the nested function
- template exists, it provides a partial, if flawed, means of checking.
- The code to do this for member function templates looks like this
- ( similar code also exists for static member function templates ):
- template
- <
- class C,
- class T
- >
- struct TestFunctionTemplate
- {
- typedef char Bad;
- struct Good { char x[2]; };
- template<T> struct helper;
- template<class U> static Good check(helper<&U::template SomeFuncTemplateName<int,long,double> > *);
- template<class U> static Bad check(...);
- static const bool value=sizeof(check<C>(0))==sizeof(Good);
- };
-
- where 'SomeFuncTemplateName' is the name of the nested function template,
- followed by some parameters to instantiate it. The 'class C' is the type of
- the enclosing class and the 'class T' is the type of the instantiated member
- function template as a member function.
- As an example if we had:
- struct AType
- {
- template<class X,class Y,class Z> double SomeFuncTemplateName(X,Y *,Z &) { return 0.0; }
- };
-
- then instantiating the above template with:
- TestFunctionTemplate
- <
- AType,
- double (AType::*)(int,long *,double &)
- >
-
- would provide a compile-time boolean value which would tell us whether the
- nested member function template exists for the particular instantiation
- provided above. Furthermore, through the use of a macro, the TTI library
- could provide the means for specifying the name of the nested member function
- template ('SomeFuncTemplateName' above) and its set of instantiated
- parameters ('int,long,double' above) for generating the template.
- So why does not the TTI library not provide at least this much functionality for
- introspecting member function templates, even if it represents a partially flawed
- way of doing so ?
- The reason is stunningly disappointing. Although the above code is perfectly correct C++
- code ( 'clang' works correctly ), two of the major C++ compilers, in all of their different
- releases, can not handle the above code correctly. Both gcc ( g++ ) and Visual C++ incorrectly
- choose the wrong 'check' function even when the correct 'check' function applies ( Comeau C++
- also fails but I am less concerned about that compiler since it is not used nearly as much as
- the other two ). All my attempts at alternatives to the above code have also failed. The problems
- with both compilers, in this regard, can be seen more easily with this snippet:
- struct AType
- {
- template<class AA> void SomeFuncTemplate() { }
- };
-
- template<class T>
- struct Test
- {
- template<T> struct helper;
- template<class U> static void check(helper<&U::template SomeFuncTemplate<int> > *) { }
- };
- int main()
- {
- Test< void (AType::*)() >::check<AType>(0);
- return 0;
- }
-
- Both compilers report compile errors with this perfectly correct code,
- gcc:
- error: no matching function for call to 'Test<void (AType::*)()>::check(int)'
- and msvc:
- error C2770: invalid explicit template argument(s) for 'void Test<T>::check(Test<T>::helper<&U::SomeFuncTemplate<int>> *)'
- There is a workaround for these compiler problems, which is to hardcode the name
- of the enclosing class, via a macro, in the generated template rather than pass it as a
- template type. In that case both compilers can handle both the member function code and
- the code snippet above correctly. In essence, when the line:
- template<class U> static void check(helper<&U::template SomeFuncTemplate<int> > *) { }
-
- gets replaced by:
- template<class U> static void check(helper<&AType::template SomeFuncTemplate<int> > *) { }
- both gcc and Visual C++ work correctly. The same goes for the 'check' line in the
- 'TestFunctionTemplate' above.
- But the workaround destroys one of the basic tenets of the TTI library, which is that
- the enclosing class be passed as a template parameter, especially as the enclosing class
- need not actually exist ( see `BOOST_TTI_MEMBER_TYPE` and the previous discussion of 'Nested Types' ),
- without producing a compiler error. So I have decided not to implement even this methodology to
- introspect nested function templates in the TTI library.
- [endsect]
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