Thread: Templates react badly to local classes?

The following program, which uses a local class object as an argument to a templated function, does not compile using gcc 4.1.1:

Code:

template<class T>
void f(T) {}

int main() {
  class A {} a;
  f(a);
}

The error is:

test.cxx: In function ‘int main()’:
test.cxx:6: error: no matching function for call to ‘f(main()::A&)’

However, if I make the class nonlocal, then it does:

Code:

template<class T>
void f(T) {}

class A {};

int main() {
  A a;
  f(a);
}

I'm new to C++, and suspect that using the local class is somehow illegal. Is this true, or is the compiler broken?

Well, in order for this to be properly defined,

Code:

template<class T>
void f(T) {}

T must not be local.

The error message may seem a little cryptic. But if you pay close attention to it, you will notice that it is not looking for 'a'. The object name is not worrying it. It's a reference to main::A that it is looking.

As such, the function instantiating is failing because the template parameter is local. At the point of definition, the function cannot find main::A

> As I understand it, this is something that ought to be allowed, but the language currently doesn't (presumably because it would be hard to implement).

Something like that, probably yes. Because the following is allowed:

Code:

template<class T>
void f(T) {}

int main() {
  int a = 12;
  f(a);
}

But then, int is not a local type.

> But then, int is not a local type.

doh!

You can't pass a local type to any other function, templated or not, IIRC. I almost never use local classes.