9 Classes [class]

9.4 Static members [class.static]

9.4.2 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 static data member in its class definition is not a definition and may be of an incomplete type other than cv-qualified void. The definition for a static data member 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 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]). A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [ Note: In both these cases, the member may appear in constant expressions.  — end note ] The member shall still be defined in a namespace scope if it is odr-used ([basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer.

Note: There shall be exactly one definition of a static data member that is odr-used ([basic.def.odr]) 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.

Static data members of a class in namespace scope have external linkage ([basic.link]). A local class shall not have static data members.

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

A static data member shall not be mutable ([dcl.stc]).