11 Classes [class]

11.4 Class members [class.mem]

11.4.9 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.
A static data member shall not be mutable ([dcl.stc]).
A static data member shall not be a direct member ([class.mem]) of an unnamed ([class.pre]) or local ([class.local]) class or of a (possibly indirectly) nested class ([class.nest]) thereof.
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.
[Note 1: 
The initializer in the definition of a static data member is in the scope of its class ([basic.scope.class]).
— end note]
[Example 1: class process { static process* run_chain; static process* running; }; process* process::running = get_main(); process* process::run_chain = running;
The definition of the static data member run_chain of class process inhabits the global scope; the notation process​::​run_chain indicates 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 2: 
Once the static data member has been defined, it exists even if no objects of its class have been created.
[Example 2: 
In the example above, run_chain and running exist even if no objects of class process are created by the program.
— end example]
The initialization and destruction of static data members is described in [basic.start.static], [basic.start.dynamic], and [basic.start.term].
— 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.
The declaration of an inline static data member (which is a definition) 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 3: 
There is exactly one definition of a static data member that is odr-used ([basic.def.odr]) in a valid program.
— end note]
[Note 4: 
Static data members of a class in namespace scope have the linkage of the name of the class ([basic.link]).
— end note]