Ambiguity between default-initialization and value-initialization

Question

I found many articles explaining the difference between "default-initialization and value-initialization" but in fact I didn't understand clearly.

Here's an example:

class A{
   public:
      int x;
};


int main(){
    A a;// default initialization so x has undefined value.
    A b = A(); // value initialization so x is a scalar thus it is value initialized to 0

}

Above it is OK as I guess but here:

int value = 4; // is this considered a value-initialization?

Please help me understand the major differences between the two forms of initializations.

Answer 1

A a; is default initialization, as the effect the default constructor of A is used for initialization. Since the implicitly-generated default constructor of A does nothing, a.x has indeterminate value.

A() is value initialization,

if T is a class type with a default constructor that is neither user-provided nor deleted (that is, it may be a class with an implicitly-defined or defaulted default constructor), the object is zero-initialized and then it is default-initialized if it has a non-trivial default constructor;

Note the difference with default initialization, A has an implicitly-defined default constructor, and the object is zero-initialized; so the data memeber x of the temporary object A() will be initialized to 0.

A b = A(); is copy initialization, in concept b is initialized from the temporary object A(), so b.x will be initialized to 0 too. Note that because of copy elision, since C++17 b is guaranteed to be value-initialized directly; the copy/move construction is omitted.

int value = 4; is copy initialization too. value will be initialized to 4.