Value initialization on explicit constructor call in C++?

Question

Possible Duplicate:
What do the following phrases mean in C++: zero-, default- and value-initialization?

There are multiple places where people have said that an explicit call to the class constructor results in value initialization [when no used-defined constructors exist] and that this is not done by the default constructor [which is a do-nothing constructor] but is something completely different.

What happens actually if no constructor is called OR What is value initialization in this case ?

Answer 1

Firstly, what happens actually if no constructor is called

A constructor for a class-type is always called when an object is constructed, be it user-defined or compiler-generated. The object is initialized, but the members can remain un-initialized. This makes the second part of the question obsolete.

Second, is there documentation that supports/mentions/explains this behaviour ?

The all-mighty standard.

Answer 2

This is only true for aggregates: Consider this:

struct Holder
{
   Aggregate a;
   NonAggr   n;

   Holder(int, char) : a(), n() { }
   Holder(char, int) { }
};

Holder h1(1, 'a');
Holder h2('b', 2);

Suppose Aggregate is an aggregate type. Now h1.a is value-initialized, which value-initializes each member, while h2.a is default-initialized, which default-initializes each member. The same holds for the n member, but if NonAggr is a non-aggregate class type, its default constructor will always be called.