static_assert in C

_Static_assert ( expression , message ) // (since C11)(deprecated in C23)
static_assert ( expression , message ) // (since C23)
_Static_assert ( expression ) // (since C23)(deprecated in C23)
static_assert ( expression ) // (since C23)

where

• expression is any integer constant expression, and
• message is any string literal

This keyword is also available as convenience macro static_assert, available in the header <assert.h> (until C23).

Both of static_assert and _Static_assert have the same effects. _Static_assert is a deprecated spelling that is kept for compatibility.

An implementation may also define static_assert and/or _Static_assert as predefined macros, and static_assert is no longer provided by <assert.h>.

Explanation

The constant expression is evaluated at compile time and compared to zero. If it compares equal to zero, a compile-time error occurs and the compiler must display message as part of the error message (except that characters not in basic character set are not required to be displayed) (until C23) should display message (if provided) as part of the error message (since C23).

Otherwise, if expression does not equal zero, nothing happens; no code is emitted.

Example

#include <assert.h> // no longer needed since C23

int main(void)
{
    // Test if math works, C23:
    static_assert((2 + 2) % 3 == 1, "Whoa dude, you knew!");
    // Pre-C23 alternative:
    _Static_assert(2 + 2 * 2 == 6, "Lucky guess!?");

    // This will produce an error at compile time.
    // static_assert(sizeof(int) < sizeof(char), "Unmet condition!");

    constexpr int _42 = 2 * 3 * 2 * 3 + 2 * 3;
    static_assert(_42 == 42); // the message string can be omitted.

    // const int _13 = 13;
    // Compile time error - not an integer constant expression:
    // static_assert(_13 == 13);
}