9 Declarations [dcl.dcl]

9.4 Initializers [dcl.init]

9.4.1 General [dcl.init.general]

The process of initialization described in [dcl.init] applies to all initializations regardless of syntactic context, including the initialization of a function parameter ([expr.call]), the initialization of a return value ([stmt.return]), or when an initializer follows a declarator.
[Note 1:
The rules in [dcl.init] apply even if the grammar permits only the brace-or-equal-initializer form of initializer in a given context.
— end note]
Except for objects declared with the constexpr specifier, for which see [dcl.constexpr], an initializer in the definition of a variable can consist of arbitrary expressions involving literals and previously declared variables and functions, regardless of the variable's storage duration.
[Example 1: int f(int); int a = 2; int b = f(a); int c(b); — end example]
[Note 2:
Default arguments are more restricted; see [dcl.fct.default].
— end note]
[Note 3:
The order of initialization of variables with static storage duration is described in [basic.start] and [stmt.dcl].
— end note]
A declaration of a block-scope variable with external or internal linkage that has an initializer is ill-formed.
To zero-initialize an object or reference of type T means:
  • if T is a scalar type, the object is initialized to the value obtained by converting the integer literal 0 (zero) to T;93
  • if T is a (possibly cv-qualified) non-union class type, its padding bits are initialized to zero bits and each non-static data member, each non-virtual base class subobject, and, if the object is not a base class subobject, each virtual base class subobject is zero-initialized;
  • if T is a (possibly cv-qualified) union type, its padding bits are initialized to zero bits and the object's first non-static named data member is zero-initialized;
  • if T is an array type, each element is zero-initialized;
  • if T is a reference type, no initialization is performed.
To default-initialize an object of type T means:
  • If T is a (possibly cv-qualified) class type ([class]), constructors are considered.
    The applicable constructors are enumerated ([over.match.ctor]), and the best one for the initializer () is chosen through overload resolution ([over.match]).
    The constructor thus selected is called, with an empty argument list, to initialize the object.
  • If T is an array type, each element is default-initialized.
  • Otherwise, no initialization is performed.
A class type T is const-default-constructible if default-initialization of T would invoke a user-provided constructor of T (not inherited from a base class) or if
  • each direct non-variant non-static data member M of T has a default member initializer or, if M is of class type X (or array thereof), X is const-default-constructible,
  • if T is a union with at least one non-static data member, exactly one variant member has a default member initializer,
  • if T is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and
  • each potentially constructed base class of T is const-default-constructible.
If a program calls for the default-initialization of an object of a const-qualified type T, T shall be a const-default-constructible class type or array thereof.
To value-initialize an object of type T means:
  • if T is a (possibly cv-qualified) class type ([class]), then
    • if T has either no default constructor ([class.default.ctor]) or a default constructor that is user-provided or deleted, then the object is default-initialized;
    • otherwise, the object is zero-initialized and the semantic constraints for default-initialization are checked, and if T has a non-trivial default constructor, the object is default-initialized;
  • if T is an array type, then each element is value-initialized;
  • otherwise, the object is zero-initialized.
A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed.
[Note 4:
For every object of static storage duration, static initialization ([basic.start.static]) is performed at program startup before any other initialization takes place.
In some cases, additional initialization is done later.
— end note]
An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.
[Note 5:
Since () is not permitted by the syntax for initializer, X a(); is not the declaration of an object of class X, but the declaration of a function taking no argument and returning an X.
The form () is permitted in certain other initialization contexts ([expr.new], [expr.type.conv], [class.base.init]).
— end note]
If no initializer is specified for an object, the object is default-initialized.
An initializer for a static member is in the scope of the member's class.
[Example 2: int a; struct X { static int a; static int b; }; int X::a = 1; int X::b = a; // X​::​b = X​::​a — end example]
If the entity being initialized does not have class type, the expression-list in a parenthesized initializer shall be a single expression.
The initialization that occurs in the = form of a brace-or-equal-initializer or condition ([stmt.select]), as well as in argument passing, function return, throwing an exception ([except.throw]), handling an exception ([except.handle]), and aggregate member initialization ([dcl.init.aggr]), is called copy-initialization.
[Note 6:
Copy-initialization may invoke a move ([class.copy.ctor]).
— end note]
The initialization that occurs is called direct-initialization.
The semantics of initializers are as follows.
The destination type is the type of the object or reference being initialized and the source type is the type of the initializer expression.
If the initializer is not a single (possibly parenthesized) expression, the source type is not defined.
An initializer-clause followed by an ellipsis is a pack expansion ([temp.variadic]).
If the initializer is a parenthesized expression-list, the expressions are evaluated in the order specified for function calls ([expr.call]).
The same identifier shall not appear in multiple designators of a designated-initializer-list.
An object whose initialization has completed is deemed to be constructed, even if the object is of non-class type or no constructor of the object's class is invoked for the initialization.
[Note 9:
Such an object might have been value-initialized or initialized by aggregate initialization ([dcl.init.aggr]) or by an inherited constructor ([class.inhctor.init]).
— end note]
Destroying an object of class type invokes the destructor of the class.
Destroying a scalar type has no effect other than ending the lifetime of the object ([basic.life]).
Destroying an array destroys each element in reverse subscript order.
A declaration that specifies the initialization of a variable, whether from an explicit initializer or by default-initialization, is called the initializing declaration of that variable.
[Note 10:
In most cases this is the defining declaration ([basic.def]) of the variable, but the initializing declaration of a non-inline static data member ([class.static.data]) might be the declaration within the class definition and not the definition at namespace scope.
— end note]
As specified in [conv.ptr], converting an integer literal whose value is 0 to a pointer type results in a null pointer value.