9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.6 The constinit specifier [dcl.constinit]

1
#
The constinit specifier shall be applied only to a declaration of a variable with static or thread storage duration.
If the specifier is applied to any declaration of a variable, it shall be applied to the initializing declaration.
No diagnostic is required if no constinit declaration is reachable at the point of the initializing declaration.
2
#
If a variable declared with the constinit specifier has dynamic initialization ([basic.start.dynamic]), the program is ill-formed.
[Note
:
The constinit specifier ensures that the variable is initialized during static initialization ([basic.start.static]).
— end note
]
3
#
[Example
: 
const char * g() { return "dynamic initialization"; }
constexpr const char * f(bool p) { return p ? "constant initializer" : g(); }
constinit const char * c = f(true);     // OK
constinit const char * d = f(false);    // error
— end example
]