Need some clarification with 8.5.p7 in the C++11 Standard

Question

Paragraph 8.5p7 of the C++11 Standard states:

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type (Clause 9) with a user-provided constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);

• if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is called.

• if T is an array type, then each element is value-initialized;

• otherwise, the object is zero-initialized.

I have a problem understanding the characters in bold above. How the additional calling of T's implicit default constructor could alter the zero-initialization, that has just occurred in this case?

Answer 1

Here's a concrete example:

class A {
    int a;
public:
    A() : a(1) {}
};

class B {
    int b;
    A c;
};

B falls in this category - it's a non-union class type without a user-provided constructor. So if a B is value-initialized, it will first be zero-initialized (so both b and c.a will be set to 0), and then the default constructor will be called (which will call A's constructor and set c.a to 1).

By the as-if rule, these might be combined into a single step by the optimizer (which will set b to 0 and c.a to 1), as no-one can ever see the object between the zero initialization and the default constructor.