C++ Zero-Initialization

Question

I\'m having trouble understanding when and why exactly a member in my class is zero-initialized according to http://en.cppreference.com/w/cpp/language/zero_initialization.

Answer 1

The following

MyTest testObj = {};

is not zero-initialization for MyTest, but is simply calling its default constructor. The cppreference page explains why (emphasis mine):

As part of value-initialization sequence for non-class types and for members of value-initialized class types that have no constructors, including value initialization of elements of aggregates for which no initializers are provided.

MyTest is a class type, and a has a constructor.

---

Defining the constructor with

MyTest() = default;

will instead zero-initialize the object.

Relevant Standard quotes (emphasis mine) below.

From [dcl.init#8]:

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type with either no default constructor ([class.ctor]) or a default constructor that is user-provided or deleted, then the object is default-initialized;

• if T is a (possibly cv-qualified) class type without a user-provided or deleted default constructor, then the object is zero-initialized and the semantic constraints for default-initialization are checked, and if T has a non-trivial default constructor, the object is default-initialized;

• ...

From [dcl.init.list]:

List-initialization of an object or reference of type T is defined as follows:

• ...

• Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.