What does 'value initializing' something mean?

Question

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

If I have a class for example:

class Info
{
   int x;
   int y;
};

which I used to created an object,

Info *p = new Info();

Does the brackets beside Info mean i'm value initializing it? How does it different from this, Info *p = new Info; ?

I know there is a question which differentiate between different meanings in new and old C++ language but I want to know the semantic difference between default and value initialization e.g. Does value initialization means initializing something to zero?

Answer 1

A declared variable can be Zero Initialized, Value Initialized or Default Initialized.

In your example:

Info *p = new Info();    <------- Value Initialization
Info *p = new Info;      <------- Default Initialization

The C++03 Standard 8.5/5 aptly defines each:

To zero-initialize an object of type T means:

— if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T;
— if T is a non-union class type, each nonstatic data member and each base-class subobject
is zero-initialized;
— if T is a union type, the object’s first named data member is zero-initialized;
— if T is an array type, each element is zero-initialized;
— if T is a reference type, no initialization is performed.

To default-initialize an object of type T means:
— if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is an array type, each element is default-initialized;
— otherwise, the object is zero-initialized.

To value-initialize an object of type T means:
— if T is a class type (clause 9) with a user-declared 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 non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized