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

Yes. In the first case you can declare _capacity, _data and _len as constants:

class MyClass
{
private:
    const int _capacity;
    const void *_data;
    const int _len;
// ...
};

This would be important if you want to ensure const-ness of these instance variables while computing their values at runtime, for example:

MyClass::MyClass() :
    _capacity(someMethod()),
    _data(someOtherMethod()),
    _len(yetAnotherMethod())
{
}

const instances must be initialized in the initializer list or the underlying types must provide public parameterless constructors (which primitive types do).