• constinit - asserts that a variable has static initialization, i.e.zero initialization andconstant initialization, otherwise the program is ill-formed.

[edit]Explanation

Theconstinit specifier declares a variable with static or threadstorage duration.

If a variable is declared withconstinit, itsinitializing declaration must be applied withconstinit. If a variable declared withconstinit hasdynamic initialization (even if it isperformed as static initialization), the program is ill-formed.

If noconstinit declaration is reachable at the point of the initializing declaration, the program is ill-formed, no diagnostic required.

constinit cannot be used together withconstexpr. When the declared variable is a reference,constinit is equivalent toconstexpr. When the declared variable is an object,constexpr mandates that the object must have static initialization and constant destruction and makes the object const-qualified, however,constinit does not mandate constant destruction and const-qualification. As a result, an object of a type which has constexpr constructors and no constexpr destructor (e.g.std::shared_ptr<T>) might be declared withconstinit but notconstexpr.

constchar* g(){return"dynamic initialization";}constexprconstchar* f(bool p){return p?"constant initializer": g();} constinitconstchar* c= f(true);// OK// constinit const char* d = f(false); // error

constinit can also be used in a non-initializing declaration to tell the compiler that athread_local variable is already initialized,reducing overhead that would otherwise be incurred by a hidden guard variable.

extern thread_local constinitint x;int f(){return x;}// no check of a guard variable needed

[edit]Notes

[edit]Keywords

constinit

[edit]Example

#include <cassert> constexprint square(int i){return i* i;} int twice(int i){return i+ i;} constinitint sq= square(2);// OK: initialization is done at compile time// constinit int x_x = twice(2); // Error: compile time initializer required int square_4_gen(){static constinitint pow= square(4); // constinit int prev = pow; // Error: constinit can only be applied to a// variable with static or thread storage durationint prev= pow;    pow= pow* pow;return prev;} int main(){assert(sq==4);    sq= twice(1);// Unlike constexpr this value can be changed later at runtimeassert(sq==2); assert(square_4_gen()==16);assert(square_4_gen()==256);assert(square_4_gen()==65536);}

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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