What is the name of the function which violates the rules of Object Oriented Programming concept?

Answer 1

There is no such function. Some will say friend functions undermine encapsulation however this is errant nonsense. All members of a class are automatically friends of all instances of the same class, and yet no-one ever claims this to undermine encapsulation.

To quote Bjarne Stroustrup himself:

"Friend" is an explicit mechanism for granting access, just like membership. You cannot (in a standard conforming program) grant yourself access to a class without modifying its source. For example:

class X {

int i;

public:

void m(); // grant X::m() access

friend void f(X&); // grant f(X&) access

// ...

};

void X::m() { i++; /* X::m() can access X::i */ }

void f(X& x) { x.i++; /* f(X&) can access X::i */ }