Template function inside template class

Question

I have this code:

template <class T>
class MyClass {
public:
    template <class U>
    void foo() {
        U a;
        a.invoke();
    }
};

I want it in this form:

template <class T>
class MyClass {
public:
    template <class U>
    void foo();
};

template <class T> /* ????? */
void MyClass<T>::foo() {
    U a;
    a.invoke();
}

How can I do this? What is the correct syntax?

Schear

Why not just do the function decl inside the class decl (see codepad.org/wxaZOMYW)? You can't move the function decl out of the header anyway, so... – Breathtaking

@hiobs: FWIW, you can move the declaration into a CPP file. That said, I've only done this once to do some hackery. In that case, knowing how to do this is essential. – Romilly

Sometimes one must move the function definition outside of the class, after definition of dependencies needed by the function body. This happens when class A uses class B and B also uses A. In that case you declare A and B, then define A and B methods. – Swept

Answer 1

Write this:

template <class T>
template <class U>
void MyClass<T>::foo() { /* ... */ }

Ecumenical

void MyClass<T>::foo<T>()... thanks, I tried it before, but it doesn't work to me.. perhaps I had to do clean project. – Schear

@user1074367: No, I think it's as I say. – Ecumenical

actually I wrote: template <class T> template <class U> void MyClass<T>::foo() { U a; a.invoke(); } and it works – Schear

@user1074367: Err... yes, that's what I say in the answer, non? – Ecumenical

sorry, you are right, my mistake... I thought, you wrote MyClass<U> – Schear

@user1074367: I'm not saying foo<U>... hit the refresh button, maybe you're seeing an earlier edit :-) – Ecumenical

I know this is old, but I can't find much more info on it... Is this a typical design that appears often or maybe hints at a bad design? – Dang

@mike: Member templates are a perfectly normal and common thing. – Ecumenical