Re: please help with friend function in nested class template
LiDongning <lidongning@gmail.com> wrote:
I want to declare a function template as a friend in a nested class
template. The code is as follows.
template <class T>
class a1
{
public:
class a2
{
T att;
template <class T> friend const typename a1<T>::a2 operator +
(const typename a1<T>::a2&, const typename a1<T>::a2&);
};
};
template <class T>
const typename a1<T>::a2 operator + (const typename a1<T>::a2&, const
typename a1<T>::a2&)
{
return a1<T>::a2 xx;
}
int main()
{
a1<int> x;
a1<int>::a2 y;
y + y; //error! c2783
system("PAUSE");
return 0;
}
Complied in VC2008. Got error C2783.
Friend declaration is a red herring. The real problem is that, in
operator+ invocation, T is in a non-deducible context. Consider:
template <typename T>
struct Outer {
struct Inner {};
};
template <typename T>
void f(const typename Outer<T>::Inner& x) {}
int main() {
Outer<int>::Inner y;
f(y); // error here
return 0;
}
In f(y) call, the compiler cannot deduce that T==int. For all it knows,
you have something like this somewhere in your program:
template<>
struct Outer<long> {
typedef Outer<int>::Inner Inner;
};
Now Outer<int>::Inner and Outer<long>::Inner both name the same type.
--
With best wishes,
Igor Tandetnik
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