11 Classes [class]

A default constructor for a class X is a constructor of class X for which each parameter that is not a function parameter pack has a default argument (including the case of a constructor with no parameters).

If there is no user-declared constructor for class X, a non-explicit constructor having no parameters is implicitly declared as defaulted ([dcl.fct.def]).

An implicitly-declared default constructor is an inline public member of its class.

A defaulted default constructor for class X is defined as deleted if:

• (2.1)
  X is a union that has a variant member with a non-trivial default constructor and no variant member of X has a default member initializer,
• (2.2)
  X is a non-union class that has a variant member M with a non-trivial default constructor and no variant member of the anonymous union containing M has a default member initializer,
• (2.3)
  any non-static data member with no default member initializer ([class.mem]) is of reference type,
• (2.4)
  any non-variant non-static data member of const-qualified type (or array thereof) with no brace-or-equal-initializer is not const-default-constructible ([dcl.init]),
• (2.5)
  X is a union and all of its variant members are of const-qualified type (or array thereof),
• (2.6)
  X is a non-union class and all members of any anonymous union member are of const-qualified type (or array thereof),
• (2.7)
  any potentially constructed subobject, except for a non-static data member with a brace-or-equal-initializer, has class type M (or array thereof) and either M has no default constructor or overload resolution ([over.match]) as applied to find M's corresponding constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor, or
• (2.8)
  any potentially constructed subobject has a type with a destructor that is deleted or inaccessible from the defaulted default constructor.

A default constructor is trivial if it is not user-provided and if:

• (3.1)
  its class has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
• (3.2)
  no non-static data member of its class has a default member initializer ([class.mem]), and
• (3.3)
  all the direct base classes of its class have trivial default constructors, and
• (3.4)
  for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.

Otherwise, the default constructor is non-trivial.

A default constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) to create an object of its class type ([intro.object]), when it is needed for constant evaluation ([expr.const]), or when it is explicitly defaulted after its first declaration.

The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with no ctor-initializer ([class.base.init]) and an empty compound-statement.

If that user-written default constructor would be ill-formed, the program is ill-formed.

If that user-written default constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined default constructor is constexpr.

Before the defaulted default constructor for a class is implicitly defined, all the non-user-provided default constructors for its base classes and its non-static data members are implicitly defined.

Default constructors are called implicitly to create class objects of static, thread, or automatic storage duration ([basic.stc.static], [basic.stc.thread], [basic.stc.auto]) defined without an initializer ([dcl.init]), are called to create class objects of dynamic storage duration ([basic.stc.dynamic]) created by a new-expression in which the new-initializer is omitted ([expr.new]), or are called when the explicit type conversion syntax ([expr.type.conv]) is used.

A program is ill-formed if the default constructor for an object is implicitly used and the constructor is not accessible ([class.access]).