11 Classes [class]

11.9 Initialization [class.init]

11.9.4 Initialization by inherited constructor [class.inhctor.init]

When a constructor for type B is invoked to initialize an object of a different type D (that is, when the constructor was inherited ([namespace.udecl])), initialization proceeds as if a defaulted default constructor were used to initialize the D object and each base class subobject from which the constructor was inherited, except that the B subobject is initialized by the invocation of the inherited constructor.

The complete initialization is considered to be a single function call; in particular, the initialization of the inherited constructor's parameters is sequenced before the initialization of any part of the D object.

[Example 1: struct B1 { B1(int, ...) { } }; struct B2 { B2(double) { } }; int get(); struct D1 : B1 { using B1::B1; // inherits B1(int, ...) int x; int y = get(); }; void test() { D1 d(2, 3, 4); // OK, B1 is initialized by calling B1(2, 3, 4), // then d.x is default-initialized (no initialization is performed), // then d.y is initialized by calling get() D1 e; // error: D1 has no default constructor } struct D2 : B2 { using B2::B2; B1 b; }; D2 f(1.0); // error: B1 has no default constructor struct W { W(int); }; struct X : virtual W { using W::W; X() = delete; }; struct Y : X { using X::X; }; struct Z : Y, virtual W { using Y::Y; }; Z z(0); // OK, initialization of Y does not invoke default constructor of X template<class T> struct Log : T { using T::T; // inherits all constructors from class T ~Log() { std::clog << "Destroying wrapper" << std::endl; } };

Class template Log wraps any class and forwards all of its constructors, while writing a message to the standard log whenever an object of class Log is destroyed.

— end example]

If the constructor was inherited from multiple base class subobjects of type B, the program is ill-formed.

[Example 2: struct A { A(int); }; struct B : A { using A::A; }; struct C1 : B { using B::B; }; struct C2 : B { using B::B; }; struct D1 : C1, C2 { using C1::C1; using C2::C2; }; struct V1 : virtual B { using B::B; }; struct V2 : virtual B { using B::B; }; struct D2 : V1, V2 { using V1::V1; using V2::V2; }; D1 d1(0); // error: ambiguous D2 d2(0); // OK, initializes virtual B base class, which initializes the A base class // then initializes the V1 and V2 base classes as if by a defaulted default constructor struct M { M(); M(int); }; struct N : M { using M::M; }; struct O : M {}; struct P : N, O { using N::N; using O::O; }; P p(0); // OK, use M(0) to initialize N's base class, // use M() to initialize O's base class — end example]

When an object is initialized by an inherited constructor, initialization of the object is complete when the initialization of all subobjects is complete.