Nested template struct constructor implementation

Question

How do I create a nested template struct implementration? For example, if I have:

foo.h:

template<class T>
class foo
{
    template<class U>
    struct bar
    {
        U u;
        bar(U u);
        ...
    };
    ...
};

and

foo.cpp

include "foo.h"
...
template<class T, class U>
foo<T>::bar<U>::bar(U u) : u(u) { }
...

I get different syntax errors such as "missing ';'", etc. What am I doing wrong?

Answer 1

There are three issues in your code:

1. You do miss semicolons as error message suggests

class definition must be followed by a semicolon:

template<class T>
class foo
{
    template<class U>
    struct bar
    {
        U u;
        bar();
        ...
    };
// ~~^ here
    ...
};
// ^ and here

2. Class foo<T>::bar<U> has no constructor which takes 1 parameter. Either change your definition or implementation.

3. template clauses are not mergeable. If you needed 2 template keywords, you still need 2 of them anywhere else:

Like this:

template<class T>
template<class U>
foo<T>::bar<U>::bar(U u) : u(u) { }

4. Maybe not really an issue, because we don't see your full code, but please read Why can templates only be implemented in the header file? before you proceed.

Answer 2

It should be

template<class T>
template<class U>
foo<T>::bar<U>::bar(U u) : u(u) { }