Forward-declaring template pointer

Question

Do I really need three statements, i.e. like this

class A;
template<class _T> class B;
typedef B<A> C;

to forward-declare a pointer of template type C, like so:

C* c = 0;

I was hoping to be able to conceal the classes A and B in my forward-declaration, is that even possible?

Answer 1

I don't really understand your question, but code like this:

template<class _T> class B;

is illegal. C++ reserves names begining with an underscore and an uppercase letter for the compiler & library implementation - you are not allowed to use them in your own code.

Answer 2

Yes you need. Note that all three lines do different things. The first declares a class. The second declares a template, and the third declares a typedef-name. In declaring the typedef-name, you can use an elaborated type specifier like "class A" to name the class without having its name in scope (with some nasty pitfalls included) - but you cannot further collapse the template declaration with the typedef declaration.

I would not worry about the three lines. If it's necessary, i would just write them out :)

Answer 3

Although not exactly the same, you could do this instead:

class C;
C* c = 0;

and then later, in the implementation file, after the header files for "A" and "B" have been included, define "C" like this:

class C : public B<A> {};

Using inheritance instead of a typedef should work if you only need to use the default constructor of B<A>.