fo.cpp

#include <iostream>

class square {};
class circle {};

template <typename T>
void f(T) { std::cout << "default template" << std::endl; }
void f(square) { std::cout << "square" << std::endl; }
void f(circle) { std::cout << "circle" << std::endl; }


int main () {
    f(square());
    f(circle());
    f(10);

    return 0;
}

// Output:
// square
// circle
// default template