constinit specifier (since C++20)

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[edit] Explanation

The constinit specifier declares a variable with static or thread storage duration. If a variable is declared with constinit, its initializing declaration must be applied with constinit. If a variable declared with constinit has dynamic initialization, the program is ill-formed. If no constinit declaration is reachable at the point of the initializing declaration, the program is ill-formed, no diagnostic required.

constinit cannot be used together with constexpr or consteval. When the declared variable is a reference, constinit is equivalent to constexpr. 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 with constinit but not constexpr.

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

constinit can also be used in a non-initializing declaration to tell the compiler that a thread_local variable is already initialized.

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

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