C++三种class constructors

class A {public:    A(int a)    : a_(a) {}    A(const A &a)    : a_(a.a_) {}    A &operator=(const A &a) {        a_ = a.a_;        return *this;    }private:    int a_;};class B: A {public:    B(int a, int b)    : A(a), b_(b) {}    B(const B &b)    : A(b), b_(b.b_) {}    B &operator=(const B &b) {        A::operator=(b);        b_ = b.b_;        return *this;    }private:    int b_;};