struct complex { complex(); complex(double); complex(double,double); }; complex sqrt(complex,complex); complex a(1); // initialized by calling complex(double) with argument 1 complex b = a; // initialized as a copy of a complex c = complex(1,2); // initialized by calling complex(double,double) with arguments 1 and 2 complex d = sqrt(b,c); // initialized by calling sqrt(complex,complex) with d as its result object complex e; // initialized by calling complex() complex f = 3; // initialized by calling complex(double) with argument 3 complex g = { 1, 2 }; // initialized by calling complex(double, double) with arguments 1 and 2— end example
complex v[6] = { 1, complex(1,2), complex(), 2 };
struct X { int i; float f; complex c; } x = { 99, 88.8, 77.7 };
ctor-initializer: : mem-initializer-list
mem-initializer-list: mem-initializer ... mem-initializer-list , mem-initializer ...
mem-initializer: mem-initializer-id ( expression-list ) mem-initializer-id braced-init-list
mem-initializer-id: class-or-decltype identifier
struct A { A(); }; typedef A global_A; struct B { }; struct C: public A, public B { C(); }; C::C(): global_A() { } // mem-initializer for base A— end example
struct A { A(); }; struct B: public virtual A { }; struct C: public A, public B { C(); }; C::C(): A() { } // error: which A?— end example
struct C { C( int ) { } // #1: non-delegating constructor C(): C(42) { } // #2: delegates to #1 C( char c ) : C(42.0) { } // #3: ill-formed due to recursion with #4 C( double d ) : C('a') { } // #4: ill-formed due to recursion with #3 };— end example
struct B1 { B1(int); /* ... */ }; struct B2 { B2(int); /* ... */ }; struct D : B1, B2 { D(int); B1 b; const int c; }; D::D(int a) : B2(a+1), B1(a+2), c(a+3), b(a+4) { /* ... */ } D d(10);— end example
struct A { A() : v(42) { } // error const int& v; };— end example
struct A { A(); }; struct B { B(int); }; struct C { C() { } // initializes members as follows: A a; // OK: calls A::A() const B b; // error: B has no default constructor int i; // OK: i has indeterminate value int j = 5; // OK: j has the value 5 };— end example
struct A { A() = default; // OK A(int v) : v(v) { } // OK const int& v = 42; // OK }; A a1; // error: ill-formed binding of temporary to reference A a2(1); // OK, unfortunately— end example
struct V { V(); V(int); }; struct A : virtual V { A(); A(int); }; struct B : virtual V { B(); B(int); }; struct C : A, B, virtual V { C(); C(int); }; A::A(int i) : V(i) { /* ... */ } B::B(int i) { /* ... */ } C::C(int i) { /* ... */ } V v(1); // use V(int) A a(2); // use V(int) B b(3); // use V() C c(4); // use V()— end example
class X { int a; int b; int i; int j; public: const int& r; X(int i): r(a), b(i), i(i), j(this->i) { } };
class A { public: A(int); }; class B : public A { int j; public: int f(); B() : A(f()), // undefined behavior: calls member function but base A not yet initialized j(f()) { } // well-defined: bases are all initialized }; class C { public: C(int); }; class D : public B, C { int i; public: D() : C(f()), // undefined behavior: calls member function but base C not yet initialized i(f()) { } // well-defined: bases are all initialized };— end example
template<class... Mixins> class X : public Mixins... { public: X(const Mixins&... mixins) : Mixins(mixins)... { } };— end 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; } };
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
struct X { int i; }; struct Y : X { Y(); }; // non-trivial struct A { int a; }; struct B : public A { int j; Y y; }; // non-trivial extern B bobj; B* pb = &bobj; // OK int* p1 = &bobj.a; // undefined behavior: refers to base class member int* p2 = &bobj.y.i; // undefined behavior: refers to member's member A* pa = &bobj; // undefined behavior: upcast to a base class type B bobj; // definition of bobj extern X xobj; int* p3 = &xobj.i; // OK, X is a trivial class X xobj;
struct W { int j; }; struct X : public virtual W { }; struct Y { int* p; X x; Y() : p(&x.j) { // undefined, x is not yet constructed } };
struct C; void no_opt(C*); struct C { int c; C() : c(0) { no_opt(this); } }; const C cobj; void no_opt(C* cptr) { int i = cobj.c * 100; // value of cobj.c is unspecified cptr->c = 1; cout << cobj.c * 100 // value of cobj.c is unspecified << '\n'; } extern struct D d; struct D { D(int a) : a(a), b(d.a) {} int a, b; }; D d = D(1); // value of d.b is unspecified— end example
struct A { }; struct B : virtual A { }; struct C : B { }; struct D : virtual A { D(A*); }; struct X { X(A*); }; struct E : C, D, X { E() : D(this), // undefined behavior: upcast from E* to A* might use path E* → D* → A* // but D is not constructed // “D((C*)this)” would be defined: E* → C* is defined because E() has started, // and C* → A* is defined because C is fully constructed X(this) {} // defined: upon construction of X, C/B/D/A sublattice is fully constructed };— end example
struct V { virtual void f(); virtual void g(); }; struct A : virtual V { virtual void f(); }; struct B : virtual V { virtual void g(); B(V*, A*); }; struct D : A, B { virtual void f(); virtual void g(); D() : B((A*)this, this) { } }; B::B(V* v, A* a) { f(); // calls V::f, not A::f g(); // calls B::g, not D::g v->g(); // v is base of B, the call is well-defined, calls B::g a->f(); // undefined behavior: a's type not a base of B }— end example
struct V { virtual void f(); }; struct A : virtual V { }; struct B : virtual V { B(V*, A*); }; struct D : A, B { D() : B((A*)this, this) { } }; B::B(V* v, A* a) { typeid(*this); // type_info for B typeid(*v); // well-defined: *v has type V, a base of B yields type_info for B typeid(*a); // undefined behavior: type A not a base of B dynamic_cast<B*>(v); // well-defined: v of type V*, V base of B results in B* dynamic_cast<B*>(a); // undefined behavior: a has type A*, A not a base of B }— end example
class Thing { public: Thing(); ~Thing(); Thing(const Thing&); }; Thing f() { Thing t; return t; } Thing t2 = f(); struct A { void *p; constexpr A(): p(this) {} }; constexpr A g() { A loc; return loc; } constexpr A a; // well-formed, a.p points to a constexpr A b = g(); // error: b.p would be dangling ([expr.const]) void h() { A c = g(); // well-formed, c.p may point to c or to an ephemeral temporary }
class Thing { public: Thing(); ~Thing(); Thing(Thing&&); private: Thing(const Thing&); }; Thing f(bool b) { Thing t; if (b) throw t; // OK: Thing(Thing&&) used (or elided) to throw t return t; // OK: Thing(Thing&&) used (or elided) to return t } Thing t2 = f(false); // OK: no extra copy/move performed, t2 constructed by call to f struct Weird { Weird(); Weird(Weird&); }; Weird g() { Weird w; return w; // OK: first overload resolution fails, second overload resolution selects Weird(Weird&) }— end example
template<class T> void g(const T&); template<class T> void f() { T x; try { T y; try { g(x); } catch (...) { if (/*...*/) throw x; // does not move throw y; // moves } g(y); } catch(...) { g(x); g(y); // error: y is not in scope } }— end example