Question

such is the code,and with the error:”illegal member initialization: ‘a’ is not a base or member”,what is the meaning of the error info,and why??

class A{
public:
int a;
};
class B:public A{

public:
B();
};

B::B():a(10){    // put  "a(10)" into the constructor body is right

}

Answer 1

By the time the constructor of the derived class is invoked, the base class must already be constructed. So it’s already too late. To see why it must be this way, consider:

class Base
{
    public:
    int i;
    Base(int q) : i(q) { ; }
};

class Middle : public Base
{
    public:
    Middle() : Base(2) { printf("i=%d\n", i); }
};

class Derived : public Middle
{
    public:
    Derived() : i(3) { ; }
}

Now, think about it. The Middle constructor has to run before the Derived constructor. And the Middle constructor ensures that i is 2. So how can the Derived constructor later re-construct it with a different value?