Question

I’ve encountered some template code that doesn’t compile, and I’m not sure I understand why. The problem seems to be related to using an inner class with out-of-line method definition. The following is a simple example:

template<typename T> 
class Outer {
public:
    struct Inner {
        T a;
        Inner(T _a) : a(_a) {}
    };
    int foo(T a);
};

template<typename T>
Inner Outer<T>::foo(T a) {         //Line 43
     Inner ret(a);
     return ret;
}

int main(int argc, char *argv[]) {
    Outer<int> out;
    return 0;
}

g++ 4.2.1 fails with the error: error: expected constructor, destructor, or type conversion before ‘Outer’ referring to line 43 marked above. The code works fine when I move the definition of foo to be inline.

I also tried replacing Inner with Outer<T>::Inner on line 43, but that did not make a difference.

Any thoughts?

Answer 1

You have two problems. First, you declared it to return an int. But secondly, you need typename Outer<T>::Inner if you want to access the Inner type outside the scope of Outer.