Tests an assertion at compile time. If the specified constant expression is false, the compiler displays the specified message and the compilation fails with error C2338; otherwise, there's no effect. New in C11.

_Static_assert is a keyword introduced in C11. static_assert is a macro, introduced in C11, that maps to the _Static_assert keyword.

_Static_assert(constant-expression, string-literal);
static_assert(constant-expression, string-literal);

constant-expression
An integral constant expression that can be evaluated at compile time. If the expression is zero (false), displays the string-literal parameter and the compilation fails with an error. If the expression is nonzero (true), then there's no effect.

string-literal
The message displayed if constant-expression evaluates to zero (false). The message must be made using the base character set of the compiler. The characters can't be multibyte or wide characters.

The _Static_assert keyword, and the static_assert macro, both test a software assertion at compile time. They can be used at global or function scope.

In contrast, the assert macro and _assert and _wassert functions test a software assertion at runtime and incur a runtime cost.

Microsoft-specific behavior

In C, when you don't include <assert.h>, the Microsoft Visual C/C++ compiler treats static_assert as a keyword that maps to _Static_assert. Using static_assert is preferred because the same code will work in both C and C++.

In the following example, static_assert and _Static_assert are used to verify how many elements are in an enum and that integers are 32 bits wide.

// requires /std:c11 or higher
#include <assert.h>

enum Items
{
    A,
    B,
    C,
    LENGTH
};

int main()
{
    // _Static_assert is a C11 keyword
    _Static_assert(LENGTH == 3, "Expected Items enum to have three elements");

    // Preferred: static_assert maps to _Static_Assert and is compatible with C++
    static_assert(sizeof(int) == 4, "Expecting 32 bit integers"); 

    return 0;
}

_STATIC_ASSERT Macro
assert macro and _assert and _wassert functions /std (Specify language standard version)