[class.inhctor.init] (original) (raw)

11 Classes [class]

11.10 Initialization [class.init]

11.10.4 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; int x;int y = get();};void test() { D1 d(2, 3, 4); D1 e; } struct D2 : B2 { using B2::B2; B1 b;}; D2 f(1.0); 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); template<class T> struct Log : T { using T::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); D2 d2(0); 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); — _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.