c++ value initialization

Question

I'm reading c++ primer 4th edition by Stanley Lipmann and I'm on page 92 about value initialisation.

I just don't understand when about value initialisation. I browsed around and I know there are also default initialisation and zero-initialisation. Can anyone explain about value initialisation?

Coming to this paragraph ..

clause a)

"Some classes does not define default constructor. We cannot initialise vector of such a type by specifying only a size, we must also specify an initial value"

I do understand the above but I find that the below contradict the above sentence.

clause b)

"Element type might be of a class type that does not define any constructors. In this case, the library still creates a value-initialised object. It does so by value-initialising each member of that object"

I don't understand the clause b.

Any help is appreciated

Answer 1

a) This is true, if the class defines other constructors - thereby suppressing generation of a default constructor.

struct Foo {
  Foo(int n) : mem(n) {}
  int mem;
};

This class can't be value-initialized.

b) If the class has no constructors defined, value-initialization will simply value-initialize all sub-objects (base classes and non-static members)

struct Foo {
  Foo() : mem(0) {}
  int mem;
};

struct Bar {
  Foo f;
};

Value-initialization of Bar simply means that the f member will be value-initialized.

See e.g. What do the following phrases mean in C++: zero-, default- and value-initialization?