Constructor initialization-list evaluation order

Question

I have a constructor that takes some arguments. I had assumed that they were constructed in the order listed, but in one case it appears they were being constructed in reverse resulting in an abort. When I reversed the arguments the program stopped aborting. This is an example of the syntax I'm using. The thing is, a_ needs to be initialized before b_ in this case. Can you guarantee the order of construction?

e.g.

class A
{
  public:
    A(OtherClass o, string x, int y) :
      a_(o), b_(a_, x, y) { }

    OtherClass a_;
    AnotherClass b_;
};

Answer 1

It depends on the order of member variable declaration in the class. So a_ will be the first one, then b_ will be the second one in your example.

Answer 2

To quote the standard, for clarification:

12.6.2.5

Initialization shall proceed in the following order:

...

• Then, nonstatic data members shall be initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).

...