12 Classes [class]

12.2 Class members [class.mem]

12.2.3 Static members [class.static] Static data members [class.static.data]

A static data member is not part of the subobjects of a class. If a static data member is declared thread_­local there is one copy of the member per thread. If a static data member is not declared thread_­local there is one copy of the data member that is shared by all the objects of the class.

The declaration of a non-inline static data member in its class definition is not a definition and may be of an incomplete type other than cv void. The definition for a static data member that is not defined inline in the class definition shall appear in a namespace scope enclosing the member's class definition. In the definition at namespace scope, the name of the static data member shall be qualified by its class name using the ​::​ operator. The initializer expression in the definition of a static data member is in the scope of its class ([basic.scope.class]). [Example:

class process {
  static process* run_chain;
  static process* running;

process* process::running = get_main();
process* process::run_chain = running;

The static data member run_­chain of class process is defined in global scope; the notation process​::​run_­chain specifies that the member run_­chain is a member of class process and in the scope of class process. In the static data member definition, the initializer expression refers to the static data member running of class process. end example]

[Note: Once the static data member has been defined, it exists even if no objects of its class have been created. [Example: In the example above, run_­chain and running exist even if no objects of class process are created by the program. end example] end note]

If a non-volatile non-inline const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression ([expr.const]). The member shall still be defined in a namespace scope if it is odr-used in the program and the namespace scope definition shall not contain an initializer. An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer. If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see [depr.static_constexpr]). Declarations of other static data members shall not specify a brace-or-equal-initializer.

[Note: There shall be exactly one definition of a static data member that is odr-used in a program; no diagnostic is required. end note] Unnamed classes and classes contained directly or indirectly within unnamed classes shall not contain static data members.

[Note: Static data members of a class in namespace scope have the linkage of that class. A local class cannot have static data members. end note]

Static data members are initialized and destroyed exactly like non-local variables ([basic.start.static], [basic.start.dynamic], [basic.start.term]).

A static data member shall not be mutable.