Nested class of templated class with friend operator, compile error

Question

When I try to compile this (g++):

template<typename T>
struct A {

    struct B { };

    template<typename S>
    friend A<S>& operator +(A<S> const &, A<S> const &);
};

template<typename T>
A<T>& operator +(A<T> const &a, A<T> const &b) {
    A<T>::B *x;
    return a;
}

main() { }

I get

test.cpp: In function "A<T>& operator+(const A<T>&, const A<T>&)":
test.cpp:12:11: error: "x" was not declared in this scope

Why?

[disregard: if i don't include this line, stack overflow says i have too much code in my post when i save]

`B` is dependent type, use `typename A::B *x;` See http://stackoverflow.com/questions/610245/where-and-why-do-i-have-to-put-the-template-and-typename-keywords — hmjd, Feb 25 '13 at 09:43
and you're using `S` in the declaration and `T` in the definition of your `operator+` — Tony The Lion, Feb 25 '13 at 09:43

score 2 · Accepted Answer · answered Feb 25 '13 at 09:44

2

The compiler doesn't know that A<T>::B denotes a type so it tries to do multiplication there.

Use typename A<T>::B *x;

answered Feb 25 '13 at 09:44

Karoly Horvath

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Thanks for the fix and the bonus explanation. – Paul Draper Feb 25 '13 at 10:05

Nested class of templated class with friend operator, compile error

1 Answers1