Both "zero-initialize" and "default-initialize" are terms that have precise definitions. In this context if you substitute the definitions it just means first

Both "zero-initialize" and "default-initialize" are terms that have precise definitions. In this context if you substitute the definitions it just means first zero-initializing the non-static data members (and zeroing the padding), then calling the default constructor. It doesn't mean that the lifetime of the object starts twice, or anything weird like that.