In C++ why can I access a function defined in a namespace without namespace qualifer

Question

I have a namesapce ns. Within ns I defined a class and a function. I further defined the function to be a friend of the class so that the function can access a private variable of the class. In the main program, I succeeded in calling the function without quflifying it with ns::. Please see the code for details.

#include <iostream>

namespace ns {
    class MyClass;
    void print_fn(const MyClass & c);
}

namespace ns {
    class MyClass {
    private:
        int _a;
    public:
        MyClass(int a): _a(a) {}

        friend void print_fn(const MyClass & c);
    };

    void print_fn(const MyClass & c){
        std::cout << c._a << std::endl;
    }
}


int main()
{
    ns::MyClass c(10);
    print_fn(c);

    return 0;
}

Answer 1

This is called Argument Dependent Lookup.

The compiler searches the function in the namespace where its arguments were declared.

You can disable the argument dependent lookup the following way

 ( print_fn )(c);

In this case the compiler issues an error that the function print_fn is not found.