Re: [boost] Re: Request: new config macro (msvc forward declarations)

From: John Maddock (john_maddock_at_[hidden])
Date: 2002-08-13 05:58:50

Sorry for the delay in getting back to you on this...

It looks to me as if the problem is with forward declarations rather than
default template argument as such, maybe we should just use BOOST_MSVC
<=1300 for this if it's very compiler specific (I stand to be corrected of
course)....

John.

> Neither :-) The issue is that I haven't found any official
> documentation that says when the problem occurs.
>
> parameters with 2 defaults, it seems (*seems*!) that, regardless of
> whether a default expression references one of the other parameters,
> the problem doesn't occur when
>
> a) either the first encountered declaration of the template is a
> definition; e.g.:
>
>
> // ok
> //
> template <class A = int, class B = A>
> class Test {};
>
>
> // ok too
> //
> template <class T>
> class traits {};
>
> template <class A = int, class B = traits<A> >
> class Test {};
>
>
> b) or the declarations and the definition all give the (same) default
> arguments:
>
> // ok
> //
> template <class A = int, class B = double>
> class Test;
>
> template <class A = int, class B = double>
> class Test {};
>
>
>
> Instead these all give an error at the point of definition ("C2984:
> 'Test' : template parameters '' and '' do not match"):
>
> // (1)
> //
> template <class A = int, class B = A>
> class Test;
>
> template <class A, class B>
> class Test {};
>
> // (2)
> //
> template <class A, class B>
> class Test;
>
> template <class A = int, class B = double>
> class Test {};
>
>
> // (3)
> //
> template <class T>
> class traits {};
>
> template <class A = int, class B = traits<A> >
> class Test;
>
> template <class A, class B>
> class Test {};
>
>
>
> Note also, that this passes:
>
>
> // 1 default argument - ok
> //
> template <class A, class B = A>
> class Test;
>
> template <class A, class B>
> class Test {};
>
>
>
>
> If the above circumscribes the problem correctly, for libraries that
> use the classical xxx_fwd.hpp there are at least 3 choices (let's call
> M a suitable macro):
>
>
> 1. not using default arguments at all for compiler(s) that have/has
> this problem
>
> // in the declaration
> # ifdef M
>     template <class A, class B>
> # else
>     template class A = int, class B = traits<A>
> #endif
>      class Test;
>
>
> 2. Using only the number of defaults that the compiler accepts (well,
> it could depend on the compiler but I don't know if there are others
> with the same bug)
>
> Example similar to 1.
>
>
> 3. specifying defaults in the definition too:
>
> // in the definition
> //
> # ifdef M
>     template <class A = int, class B = traits<A> >
> # else
>     template <class A, class B>
> # endif
>      class Test {};
>
>
> This has the draw back that eventually a change of the default
> arguments has to be done in more than one point, but I wouldn't be
> worried for that. For most libraries, changing the default arguments
> would break user code. Option 3 is also feasible without any macro, of
> course.
>
>
>
> In any case, it would be nice if someone that regularly uses VC had
> some more information about the problem. Any hints?
>
>
>
> Genny.
>
>
>
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