Can non-templated nested classes of class templates be implemented in a C++ header?

Question

Suppose a header file myheader.hxx defines a class template A in which a non-templated class B is defined (that does not depend on the template parameter of A ):

template<class T>
class A {
    class B { 
        // ...
    };
};

Is it okay in this case to implement B in myheader.hxx , or do I need to write a separate myheader.cxx to avoid duplicate definitions at link time? Is this case handeled consistently by different compilers?

Answer 1

It's still either a template (or part of template, don't know the ultra-precise definitions) even if it's not the top-level template, so you need to should implement it in the header (technically, it can be in a source file if that's the only place it's used, but that probably defeats the purpose).

Note: if you're not going to implement its member functions inline with the class definition, you need syntax like:

template<typename T>
void A<T>::B::foo(...)
{
    // ...
}

Also, because it's come up before, if B happened to have its own template parameter, it would be something like:

template<typename T>
template<typename T2>
void A<T>::B<T2>::foo(...)
{
    // ...
}

Not:

template<typename T, typename T2>
void A<T>::B<T2>::foo(...)
{
    // ...
}

Or if B didn't but B::foo did, it would be:

template<typename T>
template<typename T2>
// void A<T>::B::foo<T2>(...) // not this apparently
void A<T>::B::foo(...)
{
    // ...
}

EDIT : apparently it's foo above instead of foo<T2> for a function, at least with GCC (so almost 100% sure that's standard behavior)...I'm sure some language lawyer will be able to explain why :)

Etc.