Re: Forward declarations, templates, and typedefs

From:

"mlimber" <mlimber@gmail.com>

Newsgroups:

comp.lang.c++

Date:

2 May 2006 10:46:31 -0700

Message-ID:

<1146591991.097914.46360@g10g2000cwb.googlegroups.com>

Noah Roberts wrote:

I have something like the following:

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

typedef Generic<Val1> v1_type;
typedef Generic<Val2> v2_type;
typedef Generic<Val2, 1> incompat_v2_type;

I'm going to be making this class fundamental to the nature of a rather
large project (the goal is to start using it everywhere instead of
something that is currently done). I don't want to have to include the
header every time I have to return a value of this type. I want to
forward declare in headers and include in source files.

class v1_type; does not work.

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; }
};

Cheers! --M