Question

I have a class that uses a struct, and I want to overload the << operator for that struct, but only within the class:

typedef struct my_struct_t {
  int a;
  char c;
} my_struct;

class My_Class
{
  public:
    My_Class();
    friend ostream& operator<< (ostream& os, my_struct m);
}

I can only compile when I declare the operator<< overload w/ the friend keyword, but then the operator is overloaded everywhere in my code, not just in the class. How do I overload the << operator for my_struct ONLY within the class?

Edit: I will want to use the overloaded operator to print a my_struct which IS a member of My_Class

Answer 1

How do I overload the << operator for my_struct ONLY within the class?

Define it as

static std::ostream & operator<<( std::ostream & o, const my_struct & s ) { //...

or

namespace {
    std::ostream & operator<<( std::ostream & o, const my_struct & s ) { //...
}

in the .cpp file in which you implement MyClass.

EDIT: If you really, really need to scope on the class and nothing else, then define it as a private static function in said class. It will only be in scope in that class and it’s subclasses. It will hide all other custom operator<<‘s defined for unrelated classes, though (again, only inside the class, and it’s subclasses), unless they can be found with ADL, or are members of std::ostream already.