15 Special member functions [special]

struct complex {
  complex();
  complex(double);
  complex(double,double);
};

complex sqrt(complex,complex);

complex a(1);                   // initialize by a call of complex(double)
complex b = a;                  // initialize by a copy of a
complex c = complex(1,2);       // construct complex(1,2) using complex(double,double),
                                // copy/move it into c
complex d = sqrt(b,c);          // call sqrt(complex,complex) and copy/move the result into d
complex e;                      // initialize by a call of complex()
complex f = 3;                  // construct complex(3) using complex(double), copy/move it into f
complex g = { 1, 2 };           // initialize by a call of complex(double, double)

complex v[6] = { 1, complex(1,2), complex(), 2 };

struct X {
  int i;
  float f;
  complex c;
} x = { 99, 88.8, 77.7 };

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

struct A { A(); };
struct B: public virtual A { };
struct C: public A, public B { C(); };
C::C(): A() { }                 // ill-formed: which A?

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
};

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);

struct A {
  A() : v(42) { }   // error
  const int& v;
};

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
};

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

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()

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: 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: calls member function but base C not yet initialized
  i(f()) { }        // well-defined: bases are all initialized
};

template<class... Mixins>
class X : public Mixins... {
public:
  X(const Mixins&... mixins) : Mixins(mixins)... { }
};

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);           // 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