15 Special member functions [special]

15.6 Initialization [class.init]

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

1
#
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
2
#
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.
3
#
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
 ]
4
#
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
 ]
5
#
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.
6
#
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
 ]
7
#
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 ([intro.execution]).
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.
8
#
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
 ]
9
#
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
  • (9.1)
    if the entity is a non-static data member that has a default member initializer ([class.mem]) and either
    the entity is initialized from its default member initializer as specified in [dcl.init];
  • (9.2)
    otherwise, if the entity is an anonymous union or a variant member ([class.union.anon]), no initialization is performed;
  • (9.3)
    otherwise, the entity is default-initialized ([dcl.init]).
[ Note
:
An abstract class ([class.abstract]) 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
 ]
10
#
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
 ]
11
#
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
 ]
12
#
In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked ([class.dtor]).
[ Note
:
This provision ensures that destructors can be called for fully-constructed subobjects in case an exception is thrown ([except.ctor]).
— end note
 ]
13
#
In a non-delegating constructor, initialization proceeds in the following order:
  • (13.1)
    First, and only for the constructor of the most derived class ([intro.object]), 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.
  • (13.2)
    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).
  • (13.3)
    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).
  • (13.4)
    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
 ]
14
#
[ 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
 ]
15
#
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
 ]
16
#
Member functions (including virtual member functions, [class.virtual]) can be called for an object under construction.
Similarly, an object under construction can be the operand of the typeid operator ([expr.typeid]) or of a dynamic_­cast ([expr.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
 ]
17
#
[ 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
 ]
18
#
A mem-initializer followed by an ellipsis is a pack expansion ([temp.variadic]) 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
 ]