Initialization Order of Class Data Members

Question

In the following code, when the ctor of X is called will the ctor of A or B be called first? Does the order in which they are placed i

Answer 1

Initialization is always in the order that the class members appear in your class definition, so in your example a, then b.

There is a sequence point between the initialization of each member and you can pass a reference to a yet-to-be initialized member into the constructor of a class member but you would only be able to use it in limited ways (such as taking its address to form a pointer), other uses may well cause undefined behaviour.

Destruction of class members always happens in the reverse order of construction.

Order of initialization of bases and members is defined in 12.6.2 [class.base.init]/5.