Default, value and zero initialization mess

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C++14 specifies initialization of objects created with new in [expr.new]/17 ([expr.new]/15 in C++11, and the note wasn’t a note but normative text back then):

A new-expression that creates an object of type T initializes that
object as follows:

  • If the new-initializer is omitted, the object is default-initialized (8.5). [ Note: If no initialization is
    performed, the object has an indeterminate value. — end note ]
  • Otherwise, the new-initializer is interpreted according to the initialization rules of 8.5 for direct-initialization.

Default-initialization is defined in [dcl.init]/7 (/6 in C++11, and the wording itself has the same effect):

To default-initialize an object of type T means:

  • if T is a (possibly cv-qualified) class type (Clause 9), the default constructor (12.1) for T is called (and the initialization
    is ill-formed if T has no default constructor or overload resolution
    (13.3) results in an ambiguity or in a function that is deleted or
    inaccessible from the context of the initialization);
  • if T is an array type, each element is default-initialized;
  • otherwise, no initialization is performed.

Thus

  • new A solely causes As default constructor to be called, which does not initialize m. Indeterminate value. Should be the same for new B.

  • new A() is interpreted according to [dcl.init]/11 (/10 in C++11):

    An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.

    And now consider [dcl.init]/8 (/7 in C++11†):

    To value-initialize an object of type T means:

    • if T is a (possibly cv-qualified) class type (Clause 9) with either no default constructor (12.1) 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;
    • if T is an array type, then each element is value-initialized;
    • otherwise, the object is zero-initialized.

    Hence new A() will zero-initialize m. And this should be equivalent for A and B.

  • new C and new C() will default-initialize the object again, since the first bullet point from the last quote applies (C has a user-provided default constructor!). But, clearly, now m is initialized in the constructor in both cases.

† Well, this paragraph has slightly different wording in C++11, which does not alter the result:

To value-initialize an object of type T means:

  • if T is a (possibly cv-qualified) class type (Clause 9) with a
    user-provided 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 (possibly cv-qualified) non-union
    class type without a user-provided constructor, then the object is
    zero-initialized and, if T’s implicitly-declared default constructor
    is non-trivial, that constructor is called.
  • if T is an array type,
    then each element is value-initialized;
  • otherwise, the object is
    zero-initialized.

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