constinit specifier (since C++20)

Since C++20

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

# Notes

Feature-test macro Value Std Feature __cpp_constinit 201907L (C++20) 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

DRApplied toBehavior as publishedCorrect behavior
CWG 2543C++20the behavior was unclear if the variable declared with constinitis dynamically initialized as part of static initializationthe program is ill-formed in this case

# See also