If the operator is not overloaded, your program can not compile. This is a compile time error and there is no way to delay that until runtime.
A way around would be to not use an operator, but a function pointer. If the operation is not supported, than the function pointer could be set to 0 which you can detect at runtime.
class A {
public:
int q; // some data
typedef std::function<void(std::ostream& os, const A&)> PrinterFunc;
PrinterFunc func;
friend std::ostream& operator<<(std::ostream& os, const A& a) {
if(!a.func) {
throw "Not supported";
}
func(os,a);
return os;
}
};
A a;
a.func = [](std::ostream& os, const A& a) { os << "hello " << a.q; }
std::cout << a << std::endl; // will print
A b;
std::cout << b << std::endl; // will throw
This example uses C++11 and <functional>
. For C++03 you would have to use a "normal" function pointer.