Re: Forward declarations, templates, and typedefs

From:
"mlimber" <mlimber@gmail.com>
Newsgroups:
comp.lang.c++
Date:
2 May 2006 11:16:13 -0700
Message-ID:
<1146593773.230609.38460@i40g2000cwc.googlegroups.com>
Noah Roberts wrote:

mlimber wrote:

I'm not that great with templates and don't fully grasp the subtleties
of what I am trying to accomplish. Can I forward declare this template
and use it? I would prefer to do so based on the typedefs, not the
actual template.


You can't forward-declare a typedef because it is just an alias for
some other type, not a type itself. You can forward declare a class
template as such:

 template<class T> class SomeClassTemplate;

But, to forward declare a class template with default parameters, you
have to specify them in the forward declaration:

 template<ENUM X, int I = 0> class Generic;

Then you can include perhaps just your typedefs. You'll end up with
something like this:

 enum ENUM { A, B };

 template<ENUM X, int I = 0> class Generic;

 typedef Generic<A> SomeTypeA;
 typedef Generic<B> SomeTypeB;
 typedef Generic<A,1> SomeTypeBad;

 SomeTypeA* pa = 0;
 SomeTypeB* pb = 0;
 SomeTypeBad* pbad = 0;

 template<ENUM X, int I = 0> class Generic
 {
 public:
     enum { val = I };
     ENUM foo() const { return X; }
 };


So actually it might be best to create an "interface" header that had
the forward declaration and the typedefs and include it in the headers
of the clients. Then include the actual definition in sources.


If the forward declaration and typedefs are used widely, then sure.

I like templates but getting rid of the compile time dependencies with
them can be tough...


Sometimes it's impossible. For instance, you can't (portably)
forward-declare STL containers such as std::vector since
implementations are permitted to attach as many extra template
parameters as they like, so long as they're given defaults.

Cheers! --M

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