C++ error initializing base class data member in the derived class member initialization list

Question

I have a trivial question about C++ behavior. When I try to initialize the base class's data member using derived class's constructor initialization list, I am getting an error. But if I initialize inside the derived class constructor or if I call the base class constructor with the argument in the derived class member initialization list then there is no error. So the question is why I am getting error in the 3rd case.

class A {
protected:
   int data;
public:
   A(int i = 0) : data(i) {}
};
class B : public A {
public:
   B(int i = 0) { data = i; } /* works fine, initializing inside c'tor definition */
   B(int i = 0) : A(i) {} /* calling super class constructor with arg works fine */
   B(int i = 0) : data(i) {} /* error: class B does not have any field named data */
};
int main() {
    B obj1(7);
    B* obj2 = new B(8);    
    A* obj3 = new B(9);
    delete obj2; 
    delete obj3;
}

Wojtek Surowka Wojtek Surowka · Accepted Answer · 2014-06-06T10:37:07

This is so because you cannot initialise base class member in a derived class. What you call "initialisation inside the constructor" is not initialisation, it is assignment after the member was already initialised by the constructor of the base class.

C++ error initializing base class data member in the derived class member initialization list

2 Answers