9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.7 The constinit specifier [dcl.constinit]

1
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The constinit specifier shall be applied only to a declaration of a variable with static or thread storage duration or to a structured binding declaration ([dcl.struct.bind]).
[Note 1: 
A structured binding declaration introduces a uniquely named variable, to which the constinit specifier applies.
— end note]
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.
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If a variable declared with the constinit specifier has dynamic initialization ([basic.start.dynamic]), the program is ill-formed, even if the implementation would perform that initialization as a static initialization ([basic.start.static]).
[Note 2: 
The constinit specifier ensures that the variable is initialized during static initialization.
— end note]
3
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[Example 1: 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]