why private value of the obj can be changed by class instance?

Question

#include
using namespace std;

class A
{
    private:
        int value;
    public:
        A(int init):value(init){}
        void changevalue(A &am         


        

Answer 1

private doesn't mean "private to the object identity" but "private to the type (and friends)".

Note that accessibility and being able to write to the type are orthogonal concepts. You can always access a private value inside an object of your own type, but whether you can write to it or not depends on if the object is declared as const:

void f(A& a){ a.value = 4; } // OK: 'a' is not 'const'
void g(A const& a){ a.value = 4 } // error: 'a' is marked as ' const'