In this specific case, is there a difference between using a member initializer list and assigning values in a constructor?

Question

Internally and about the generated code, is there a really difference between :

MyClass::MyClass(): _capacity(15), _data(NULL), _len(0)
{
}

Answer 1

There is only one way to initialize base class instances and non-static member variables and that is using the initializer list.

If you don't specify a base or non-static member variable in your constructor's initializer list then that member or base will either be default-initialized (if the member/base is a non-POD class type or array of non-POD class types) or left uninitialized otherwise.

Once the constructor body is entered, all bases or members will have been initialized or left uninitialized (i.e. they will have an indeterminate value). There is no opportunity in the constructor body to influence how they should be initialized.

You may be able to assign new values to members in the constructor body but it is not possible to assign to const members or members of class type which have been made non-assignable and it is not possible to rebind references.

For built in types and some user-defined types, assigning in the constructor body may have exactly the same effect as initializing with the same value in the initializer list.

If you fail to name a member or base in an initializer list and that entity is a reference, has class type with no accessible user-declared default constructor, is const qualified and has POD type or is a POD class type or array of POD class type containing a const qualified member (directly or indirectly) then the program is ill-formed.