15 Special member functions [special]

15.6 Initialization [class.init]

When no initializer is specified for an object of (possibly cv-qualified) class type (or array thereof), or the initializer has the form (), the object is initialized as specified in [dcl.init].

An object of class type (or array thereof) can be explicitly initialized; see [class.expl.init] and [class.base.init].

When an array of class objects is initialized (either explicitly or implicitly) and the elements are initialized by constructor, the constructor shall be called for each element of the array, following the subscript order; see [dcl.array]. [Note: Destructors for the array elements are called in reverse order of their construction. end note]

15.6.1 Explicit initialization [class.expl.init]

An object of class type can be initialized with a parenthesized expression-list, where the expression-list is construed as an argument list for a constructor that is called to initialize the object. Alternatively, a single assignment-expression can be specified as an initializer using the = form of initialization. Either direct-initialization semantics or copy-initialization semantics apply; see [dcl.init]. [Example:

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)

end example] [Note: Overloading of the assignment operator has no effect on initialization. end note]

An object of class type can also be initialized by a braced-init-list. List-initialization semantics apply; see [dcl.init] and [dcl.init.list]. [Example:

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

Here, complex​::​complex(double) is called for the initialization of v[0] and v[3], complex​::​complex(​double, double) is called for the initialization of v[1], complex​::​complex() is called for the initialization v[2], v[4], and v[5]. For another example,

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

Here, x.i is initialized with 99, x.f is initialized with 88.8, and complex​::​complex(double) is called for the initialization of x.c. end example] [Note: Braces can be elided in the initializer-list for any aggregate, even if the aggregate has members of a class type with user-defined type conversions; see [dcl.init.aggr]. end note]

[Note: If T is a class type with no default constructor, any declaration of an object of type T (or array thereof) is ill-formed if no initializer is explicitly specified (see [class.init] and [dcl.init]). end note]

[Note: The order in which objects with static or thread storage duration are initialized is described in [basic.start.dynamic] and [stmt.dcl]. end note]

15.6.2 Initializing bases and members [class.base.init]

In the definition of a constructor for a class, initializers for direct and virtual base class subobjects and non-static data members can be specified by a ctor-initializer, which has the form

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

In a mem-initializer-id an initial unqualified identifier is looked up in the scope of the constructor's class and, if not found in that scope, it is looked up in the scope containing the constructor's definition. [Note: If the constructor's class contains a member with the same name as a direct or virtual base class of the class, a mem-initializer-id naming the member or base class and composed of a single identifier refers to the class member. A mem-initializer-id for the hidden base class may be specified using a qualified name. end note] Unless the mem-initializer-id names the constructor's class, a non-static data member of the constructor's class, or a direct or virtual base of that class, the mem-initializer is ill-formed.

A mem-initializer-list can initialize a base class using any class-or-decltype that denotes that base class type. [Example:

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]

If a mem-initializer-id is ambiguous because it designates both a direct non-virtual base class and an inherited virtual base class, the mem-initializer is ill-formed. [Example:

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

end example]

A ctor-initializer may initialize a variant member of the constructor's class. If a ctor-initializer specifies more than one mem-initializer for the same member or for the same base class, the ctor-initializer is ill-formed.

A mem-initializer-list can delegate to another constructor of the constructor's class using any class-or-decltype that denotes the constructor's class itself. If a mem-initializer-id designates the constructor's class, it shall be the only mem-initializer; the constructor is a delegating constructor, and the constructor selected by the mem-initializer is the target constructor. The target constructor is selected by overload resolution. Once the target constructor returns, the body of the delegating constructor is executed. If a constructor delegates to itself directly or indirectly, the program is ill-formed, no diagnostic required. [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]

The expression-list or braced-init-list in a mem-initializer is used to initialize the designated subobject (or, in the case of a delegating constructor, the complete class object) according to the initialization rules of [dcl.init] for direct-initialization. [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] [Note: The initialization performed by each mem-initializer constitutes a full-expression. Any expression in a mem-initializer is evaluated as part of the full-expression that performs the initialization. end note] A mem-initializer where the mem-initializer-id denotes a virtual base class is ignored during execution of a constructor of any class that is not the most derived class.

A temporary expression bound to a reference member in a mem-initializer is ill-formed. [Example:

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

end example]

In a non-delegating constructor, if a given potentially constructed subobject is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then

[Note: An abstract class is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers may be omitted. end note] An attempt to initialize more than one non-static data member of a union renders the program ill-formed. [Note: After the call to a constructor for class X for an object with automatic or dynamic storage duration has completed, if the constructor was not invoked as part of value-initialization and a member of X is neither initialized nor given a value during execution of the compound-statement of the body of the constructor, the member has an indeterminate value. end note] [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]

If a given non-static data member has both a default member initializer and a mem-initializer, the initialization specified by the mem-initializer is performed, and the non-static data member's default member initializer is ignored. [Example: Given

struct A {
  int i = /* some integer expression with side effects */ ;
  A(int arg) : i(arg) { }
  // ...
};

the A(int) constructor will simply initialize i to the value of arg, and the side effects in i's default member initializer will not take place. end example]

A temporary expression bound to a reference member from a default member initializer is ill-formed. [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]

In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked. [Note: This provision ensures that destructors can be called for fully-constructed subobjects in case an exception is thrown ([except.ctor]). end note]

In a non-delegating constructor, initialization proceeds in the following order:

  • First, and only for the constructor of the most derived class, virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.

  • Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).

  • Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).

  • Finally, the compound-statement of the constructor body is executed.

[Note: The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. end note]

[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]

Names in the expression-list or braced-init-list of a mem-initializer are evaluated in the scope of the constructor for which the mem-initializer is specified. [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) { }
};

initializes X​::​r to refer to X​::​a, initializes X​::​b with the value of the constructor parameter i, initializes X​::​i with the value of the constructor parameter i, and initializes X​::​j with the value of X​::​i; this takes place each time an object of class X is created. end example] [Note: Because the mem-initializer are evaluated in the scope of the constructor, the this pointer can be used in the expression-list of a mem-initializer to refer to the object being initialized. end note]

Member functions (including virtual member functions) can be called for an object under construction. Similarly, an object under construction can be the operand of the typeid operator or of a dynamic_­cast. However, if these operations are performed in a ctor-initializer (or in a function called directly or indirectly from a ctor-initializer) before all the mem-initializers for base classes have completed, the program has undefined behavior. [Example:

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
};

end example]

[Note: [class.cdtor] describes the result of virtual function calls, typeid and dynamic_­casts during construction for the well-defined cases; that is, describes the polymorphic behavior of an object under construction. end note]

A mem-initializer followed by an ellipsis is a pack expansion that initializes the base classes specified by a pack expansion in the base-specifier-list for the class. [Example:

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

end example]

15.6.3 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), 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.