15 Special member functions [special]

When a constructor for type B is invoked to initialize an object of a different type D (that is, when the constructor was inherited), 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:

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 a deleted default constructor
}

struct D2 : B2 {
  using B2::B2;
  B1 b;
};

D2 f(1.0);          // error: B1 has a deleted 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:

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);       // ill-formed: 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.