tmp/tmpk2vxdrim/{from.md → to.md}
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| 1 |
+
### Initialization by inherited constructor <a id="class.inhctor.init">[[class.inhctor.init]]</a>
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When a constructor for type `B` is invoked to initialize an object of a
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different type `D` (that is, when the constructor was inherited (
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[[namespace.udecl]])), initialization proceeds as if a defaulted default
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constructor were used to initialize the `D` object and each base class
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subobject from which the constructor was inherited, except that the `B`
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subobject is initialized by the invocation of the inherited constructor.
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The complete initialization is considered to be a single function call;
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in particular, the initialization of the inherited constructor’s
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parameters is sequenced before the initialization of any part of the `D`
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object.
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[*Example 1*:
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``` cpp
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struct B1 {
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B1(int, ...) { }
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};
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struct B2 {
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B2(double) { }
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};
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int get();
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struct D1 : B1 {
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using B1::B1; // inherits B1(int, ...)
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int x;
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int y = get();
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};
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void test() {
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D1 d(2, 3, 4); // OK: B1 is initialized by calling B1(2, 3, 4),
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// then d.x is default-initialized (no initialization is performed),
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// then d.y is initialized by calling get()
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D1 e; // error: D1 has a deleted default constructor
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}
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struct D2 : B2 {
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using B2::B2;
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B1 b;
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};
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D2 f(1.0); // error: B1 has a deleted default constructor
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struct W { W(int); };
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struct X : virtual W { using W::W; X() = delete; };
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struct Y : X { using X::X; };
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struct Z : Y, virtual W { using Y::Y; };
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Z z(0); // OK: initialization of Y does not invoke default constructor of X
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template<class T> struct Log : T {
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using T::T; // inherits all constructors from class T
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~Log() { std::clog << "Destroying wrapper" << std::endl; }
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};
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```
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Class template `Log` wraps any class and forwards all of its
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constructors, while writing a message to the standard log whenever an
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object of class `Log` is destroyed.
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— *end example*]
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If the constructor was inherited from multiple base class subobjects of
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type `B`, the program is ill-formed.
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[*Example 2*:
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``` cpp
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struct A { A(int); };
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struct B : A { using A::A; };
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struct C1 : B { using B::B; };
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struct C2 : B { using B::B; };
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struct D1 : C1, C2 {
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using C1::C1;
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using C2::C2;
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};
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struct V1 : virtual B { using B::B; };
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struct V2 : virtual B { using B::B; };
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struct D2 : V1, V2 {
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using V1::V1;
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using V2::V2;
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};
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D1 d1(0); // ill-formed: ambiguous
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D2 d2(0); // OK: initializes virtual B base class, which initializes the A base class
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// then initializes the V1 and V2 base classes as if by a defaulted default constructor
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struct M { M(); M(int); };
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struct N : M { using M::M; };
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struct O : M {};
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struct P : N, O { using N::N; using O::O; };
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P p(0); // OK: use M(0) to initialize N's base class,
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// use M() to initialize O's base class
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```
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— *end example*]
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When an object is initialized by an inherited constructor,
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initialization of the object is complete when the initialization of all
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| 106 |
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subobjects is complete.
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