Is there any difference if we define friend function inside or outside of class

Question

What is the difference between defining friend function inside the class or declaring inside and define outside of the class. Also why it is possible to place definition inside the class, as friend function are not member of the class.

Answer 1

There are subtle implications of the following regarding member access.

C++11 §11.4/5

A friend function defined in a class is in the (lexical) scope of the class in which it is defined. A friend function defined outside the class is not (3.4.1).

Still there as of C++17 §14.3/7

Such a function is implicitly an inline function (10.1.6). A friend function defined in a class is in the (lexical) scope of the class in which it is defined. A friend function defined outside the class is not (6.4.1).

Consider the condenced example from cppreference [Friend function definition], where f1 finds class static member and f2 finds global variable.

int i = 3;
struct X {
    friend void f1(int x) {
        i = x; // finds and modifies X::i
    }
    friend inline void f2(int);
    static const int i = 2;
};

inline void f2(int x) {
    i = x; // finds and modifies ::i
}

Of course this can't affect design desicions for the friend function. The main consideration between choices is a difference in name look up as already mentioned in the other answer. Don't forget the inline for f2 to match those of f1 implied by default.