Re: Best practices for forward declaring a template function

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Sun, 29 Nov 2009 21:40:52 +0100
Message-ID:
<heum7u$caf$1@news.eternal-september.org>
* er:

Let's begin with a library that defines:

// A_HPP
namespace library{
   class A{};
   A::value_type foo(const A& a){...}
}
// B_HPP
namespace library{
   class B{};
   B::value_type foo(const B& b){...}

}


value_type is undefined.

Assuming you meant to define on in both class A and B.

and I want to extend it, but non intrusively i.e. outside of namespace
library. I proceed like this:

template<typename T> struct is_library : mpl::false_{};

and for each X in {A,B},

// FWD_A_HPP
#include A_HPP
template<> struct is_library<X> : mpl::true_;

// FWD_FOO
namespace extension{

   typename lazy_enable_if<is_library<X>,mpl::identity<typename
X::value_type> >::type
   foo(const X& x){
     return library::foo(x)
   }

}


What's the point?

And I then define my own classes :
// C_HPP
namespace extension{
  struct C{};
  typename C::value_type foo(const C& c){...};
}


value_type is not defined.

There is no relation to the preceding explanation.

The limitation of the above, to use it say in main.cpp, is that I have
to

#include FWD_A_HPP

before

#include FWD_FOO

Correct?


No.

If the number of dependencies is large, keeping track of which files
comes before other becomes tedious so I guess the standard solution is
to add a forwarding declaration inside FWD_FOO,

namespace library{
   typename A::value_type foo(const A& a);
}

just before extension::foo. Correct?


Doesn't make sense so far.

The problem is further complicated by the fact that A, B and C, in my
actual problem, have template e.g.
template<typename T> struct A;
template<typename T,typename U> struct B;


So, now you're saying that all you've described so far has been *different* from
your problem.

but the function signature is always
templat<typename X>
typename X::value_type foo(const X&);

On some compilers (MSVC), the above does not work. I get a compile
error : ambiguous call to overloaded function.


Error on line 42.

What is the best practice for this kind of problem?


Check out the FAQ item on how to post a question about code that Does Not Work.

Cheers & hth.,

- Alf

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