Access specifier example in inheritance

inheritance.cpp

#include <iostream>

class parent
{
protected:
    int a;

public:
    parent(const int a) : a(a) {}
    bool is_equal(parent &p) { return p.a == a; } /* Can access protected (even private) member data in same class (p.a) */
};

class derived : public parent
{
public:
    derived(const int a) : parent(a) {} /* Call constructor of the parent class */
    bool is_equal(parent &p) { return p.a == a; } /* Error: Cannot access to protected member data (p.a) */
};

int main(void)
{
    parent a(2);
    derived b(2);
    std::cout << b.is_equal(a) << std::endl;

    return 0;
}