Note: C1X is a new version of the C programming language
standard. IBM continues to develop and implement the features of the
new standard. The implementation of the language level is based on
IBM's interpretation of the standard. Until IBM's implementation of
all the features of the C1X standard is complete, including the support
of a new C standard library, the implementation may change from release
to release. IBM makes no attempt to maintain compatibility, in source,
binary, or listings and other compiler interfaces, with earlier releases
of IBM's implementation of the new features of the C1X standard and
therefore they should not be relied on as a stable programming interface.
Static assertions can be declared
to detect and diagnose common usage errors at compile time. A _Static_assert declaration takes the following form:
_Static_assert declaration syntax >>-_Static_assert--(--constant_expression--,--string_literal----> >--)--;--------------------------------------------------------><
The constant_expression must be an integer constant expression. If the integer constant expression evaluates to 0, the compiler issues a severe error containing the string literal with the source location of the _Static_assert declaration. Otherwise, the _Static_assert declaration has no effect.
You can declare static assertions anywhere that you use a using declaration, including namespace scope, block scope, and class member declaration lists.
The declaration of static assertions does not declare a new type or object, and does not imply any size or time cost at run time.
static_assert is a macro defined in assert.h for C.
The addition of static assertions to the C language
has the following benefits:
- Libraries can detect common usage errors at compile time.
- Implementations of the C Standard Library can detect and diagnose common usage errors, improving usability.
Examples: _Static_assert declaration
Example 1: The following example demonstrates the use of a _Static_assert declaration inside a structure.
#include <stddef.h>
struct __attribute__((packed)) B{
char a;
int i;
};
struct A{
struct B b;
_Static_assert(offsetof(struct B,i)==1,"S not packed");
};Example 2: The following example contains
static assertions declared with static_assert, so
the assert.h header file must be included.
/* static_assert requires <assert.h> */
#include <assert.h>
static_assert(sizeof(long) >= 8, "64-bit not enabled.");Example 3: The following example shows the use of a _Static_assert declaration with an invalid constant expression.
_Static_assert(1 / 0, "never shows up!");