Order of member initialization list [duplicate]

Answer 1

Initialization of member variables occurs in the order that they are declared in the class.

http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-order

and:

http://en.cppreference.com/w/cpp/language/initializer_list

3) Then, non-static data members are initialized in order of declaration in the class definition.

Answer 2

The order how you specify in the member initializer list doesn't matter, non-static members are initialized in order of declaration in the class definition.

The order of member initializers in the list is irrelevant: the actual order of initialization is as follows:

3) Then, non-static data members are initialized in order of declaration in the class definition.

That means _b2 will always be initialized before _b; and when _b is used to initialize _b2 it's still not initialized.

BTW: The similar rule is applied for the initializations of direct base classes.

2) Then, direct base classes are initialized in left-to-right order as they appear in this class's base-specifier list