Question

Possible Duplicate:
How to use local classes with templates?

g++ 4.4 is refusing to compile a call to a template function taking a function-local class as a template parameter. Like so:

// Given this:
template <typename C>
int f(const C& c) {
  return c.g();
}

// This compiles fine:
struct C1 {
    int g() const { return 42; }
};

int h1() {
    return f(C1());
}

// But this doesn't:
int h2() {
    struct C2 {
        int g() const { return 42; }
    };
    return f(C2()); // error: no matching function for call to "f(h2()::C2)"
}

// Nor does this:
int h3() {
    struct C3 {
        int g() const { return 42; }
    };
    return f<C3>(C3()); // same error
}

What gives? How do I make this work? (In the real program from which this is pruned, “h” is a member function, and “C” has to be a nested class so that it’s implicitly a friend of the class of which “h” is a member.)

Answer 1

C++0x will remove this undesirable restriction.

For now, you can make C i a proper nested class (inside of h‘s class, not inside of h).