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 · Accepted Answer · 2014-06-06 10:37:07Z

4

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.

answered Jun 6, 2014 at 10:37

Wojtek Surowka

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Comments

isarandi · Accepted Answer · 2014-06-06 10:40:06Z

0

data is already initialized by A's constructor when we get to the init list of B (the part after the base constructor call if that exists). You cannot initialize something twice. You can either tell A's constructor how to initialize it, or assign something else to the already initialized member.

answered Jun 6, 2014 at 10:40

isarandi

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2 Comments

Ibrahim Quraish Over a year ago

So when you explicitly call the base class constructor with the argument in the derived's member initialization list, is it not initializing the same data member another time? as the A's constructor would have already been called before reaching this space. Does it not look like initializing twice?

isarandi Over a year ago

@IbrahimQuraish No. You say "A's constructor would have already been called before reaching this space" but that's false. If you explicitly call it, then A's constructor will run with the parameters that you provide and then the rest of the init list is done.

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