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

#include <cassert>

constexpr int square(int i)
{
    return i * i;
}

int twice(int i)
{
    return i + i;
}

constinit int 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 constinit int pow = square(4);
    
    // constinit int prev = pow; // Error: constinit can only be applied to a
                                 // variable with static or thread storage duration
    int prev = pow;
    pow = pow * pow;
    return prev;
}

int main()
{
    assert(sq == 4);
    sq = twice(1); // Unlike constexpr this value can be changed later at runtime
    assert(sq == 2);
    
    assert(square_4_gen() == 16);
    assert(square_4_gen() == 256);
    assert(square_4_gen() == 65536);
}

Defect reports

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

See also