constinit specifier (since C++20)

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

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 (even if it is performed as static 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. 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, reducing overhead that would otherwise be incurred by a hidden guard variable.

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

Notes

Keywords

constinit.

Example

Defect reports

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

See also