C++ zero initialization - Why is `b` in this program uninitialized, but `a` is initialized?

Question

According to the accepted (and only) answer for this Stack Overflow question,

Defining the constructor with

MyTest() = default;
         


        

Answer 1

The difference in behaviour comes from the fact that, according to [dcl.fct.def.default]/5, bar::bar is user-provided where foo::foo is not¹. As a consequence, foo::foo will value-initialize its members (meaning: zero-initialize foo::a) but bar::bar will stay uninitialized².

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¹⁾ [dcl.fct.def.default]/5

A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration.

²⁾

From [dcl.init#6]:

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.

From Vittorio Romeo's answer