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 ([basic.def.odr]) 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 ([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.
[ Note: Static data members of a class in namespace scope have the linkage of that class ([basic.link]). A local class cannot have static data members ([class.local]). — end note ]
Static data members are initialized and destroyed exactly like non-local variables ([basic.start.static], [basic.start.dynamic], [basic.start.term]).