Question

#include <iostream>
class A
{
public:
    A() { std::cout << " A ctor" << std::endl; }
    A(int i) { std::cout << " A ctor i" << std::endl; }
    ~A() { std::cout << " A dtor" << std::endl; }
};
class B: public A
{
public:
    B() : A () { std::cout << " B ctor" << std::endl; }
    ~B() { std::cout << " B dtor" << std::endl; }
};
class C: public A
{
public:
    B _b;

    C() : _b (), A () { std::cout << " C ctor" << std::endl; }
    ~C() { std::cout << " C dtor" << std::endl; }
};
int main ()
{
    C c;
}

The output is:

A ctor
A ctor
B ctor
C ctor
C dtor
B dtor
A dtor
A dtor

What is the order of the init. list? Why, in the init. list of C, ctor of A called before ctor of B? I thought the output should be:

A ctor
B ctor
A ctor
C ctor
C dtor
A dtor
B dtor
A dtor

Thanks.

Answer 1

The order in which you write initializations in the initialization list is not important, the order of initialization is determined independently of that list by other rules:

• First the base class is initialized. That’s why in the construction of C the base class constructor A is called first. Everything that belongs to the base class is constructed in this step (base classes and member variables belonging to the base class), just like when a normal object of that base class would be constructed.
• Then the member variables of the derived class are initialized, in the order in which they are declared in the class. So if there are several member variables, the order in which they are declared determines the order in which they are initialized. The order of an initialization list is not important.