A puzzle while learning Templates in C++ - c++

Question

template <class A,class B>
class H {
public:
A a;
B b;
void fac();
};
template <class B,class A>
void H<B,A>::fac() {
}
What does which follows H exactly mean ?
Since I declared template <class B,class A>, <B,A> seemed to be meaningless....
Can anybody help me ?? Thnx!!

Answer 1

There's no language rule preventing you from doing something like
template<class B> void H<B, B>::fac() {
}
and indeed it may be necessary for any explicit or partial specializations. However, since the vast, vast majority of template code is directly inline, it's really of little consequence.
For example:
template<class A, class B>
class H
{
public:
A a;
B b;
void fac();
};
template<class A> class H<A, A> {
public:
A a, b;
void fac();
};
template<class A, class B> void H<A, B>::fac() {
}
template<class B> void H<B, B>::fac() {
}

Answer 2

Typically, you put the template implementation in the template declaration itself:
template<class A, class B>
class H
{
public:
A a;
B b;
void fac()
{
// Implementation
}
};
However, it is also possible to separate it out:
template<class A, class B>
class H
{
public:
A a;
B b;
void fac();
};
template<class A, class B>
void H<A, B>::fac()
{
// Implementation
}
These two snippets mean the same thing. It's just how the C++ syntax works. Note that the identifier H in the first code snippet doesn't refer to an actual class; it refers to a template class, which is different from a normal class. H is the identifier of a family of classes that are similar but uses different template parameters. That's why you have to say H<A, B> and not just H.
Think about how it usually works for non-template classes:
class NonTemplateH
{
public:
int a;
short b;
void fac();
};
void NonTemplateH::fac()
{
// Implementation
}
You can see how they are similar.

Answer 3

The difference between
template <class A, class B>
void H<A, B>::foo()
and
template <class A, class B>
void H::foo()
is that in the second case foo is a function template inside a non-template class H.
You can only use H without the template arguments at the class scope of H. That part of the definition still belongs to the global scope, where you have to explicitly say what H is.

Answer 4

You asked:
Since I declared template <class B,class A>, <B,A> seemed to be
meaningless....
No, that is not meaningless. To understand that, consider this code:
template <class A,class B>
class H {
template<class C> //note this line!
void fac();
};
Note: Now, fac() is a function template. You've to define it like this:
template <class A,class B> //as usual.
template <class C> //notice this line especially! (not so usual)
void H<A,B>::fac()
{
//your code
}
H<A,B> is needed because it tells the compiler that A and B are template parameters to the class template H, NOT to the function template fac(). C is the tempate parameter to the function template. Notice, I didn't write fac<C>() in the definition. Compiler automacally understands it.
Also, you can interchange the symbol A and B. But if you do this, then do this consistently.