c++ member initialization list completeness

Question

must class member initialization lists in c++ be complete? or can they simply initialize one or two of the member data in a class?

thanks in advance!

Answer 1

They don't have to be complete. You can leave out base classes and non-POD class types that are default constructible, POD-types however will be left uninitialized.
Obviously constant members and references have to be initialized in the member initialization list.

Answer 2

int does have a constructor, see Stroustrup The C++ Programming Language section 6.2.8

int j = int(); 

This initializes j to 0 (The value of an explicit use of the constructor for a built-in type is 0 converted to that type, thus int() is another way of writing 0. Default constructors for built-in types are important so that templates can invoke default constructors without worry, even for built in types.