问题
I'm tidying up some older code that uses 'magic numbers' all over the place to set hardware registers, and I would like to use constants instead of these numbers to make the code somewhat more expressive (in fact they will map to the names/values used to document the registers).
However, I'm concerned that with the volume of changes I might break the magic numbers. Here is a simplified example (the register set is more complex):
const short mode0 = 0;
const short mode1 = 1;
const short mode2 = 2;
const short state0 = 0;
const short state1 = 4;
const short state2 = 8;
so instead of :
set_register(5);
we have:
set_register(state1|mode1);
What I'm looking for is a build time version of:
ASSERT(5==(state1|mode1));
Update
@Christian, thanks for the quick response, I'm interested on a C / non-boost environment answer too because this is driver/kernel code.
回答1:
NEW ANSWER :
In my original answer (below), I had to have two different macros to support assertions in a function scope and at the global scope. I wondered if it was possible to come up with a single solution that would work in both scopes.
I was able to find a solution that worked for Visual Studio and Comeau compilers using extern character arrays. But I was able to find a more complex solution that works for GCC. But GCC's solution doesn't work for Visual Studio. :( But adding a '#ifdef __ GNUC __', it's easy to choose the right set of macros for a given compiler.
Solution:
#ifdef __GNUC__
#define STATIC_ASSERT_HELPER(expr, msg) \
(!!sizeof \ (struct { unsigned int STATIC_ASSERTION__##msg: (expr) ? 1 : -1; }))
#define STATIC_ASSERT(expr, msg) \
extern int (*assert_function__(void)) [STATIC_ASSERT_HELPER(expr, msg)]
#else
#define STATIC_ASSERT(expr, msg) \
extern char STATIC_ASSERTION__##msg[1]; \
extern char STATIC_ASSERTION__##msg[(expr)?1:2]
#endif /* #ifdef __GNUC__ */
Here are the error messages reported for STATIC_ASSERT(1==1, test_message); at line 22 of test.c:
GCC:
line 22: error: negative width in bit-field `STATIC_ASSERTION__test_message'
Visual Studio:
test.c(22) : error C2369: 'STATIC_ASSERTION__test_message' : redefinition; different subscripts
test.c(22) : see declaration of 'STATIC_ASSERTION__test_message'
Comeau:
line 22: error: declaration is incompatible with
"char STATIC_ASSERTION__test_message[1]" (declared at line 22)
ORIGINAL ANSWER :
I do something very similar to what Checkers does. But I include a message that'll show up in many compilers:
#define STATIC_ASSERT(expr, msg) \
{ \
char STATIC_ASSERTION__##msg[(expr)?1:-1]; \
(void)STATIC_ASSERTION__##msg[0]; \
}
And for doing something at the global scope (outside a function) use this:
#define GLOBAL_STATIC_ASSERT(expr, msg) \
extern char STATIC_ASSERTION__##msg[1]; \
extern char STATIC_ASSERTION__##msg[(expr)?1:2]
回答2:
There is an article by Ralf Holly that examines different options for static asserts in C.
He presents three different approaches:
- switch case values must be unique
- arrays must not have negative dimensions
- division by zero for constant expressions
His conclusion for the best implementation is this:
#define assert_static(e) \
do { \
enum { assert_static__ = 1/(e) }; \
} while (0)
回答3:
Checkout boost's static assert
回答4:
You can roll your own static assert if you don't have access to a third-party library static assert function (like boost):
#define STATIC_ASSERT(x) \
do { \
const static char dummy[(x)?1:-1] = {0};\
} while(0)
The downside is, of course, that error message is not going to be very helpful, but at least, it will give you the line number.
回答5:
#define static_assert(expr) \
int __static_assert(int static_assert_failed[(expr)?1:-1])
It can be used anywhere, any times. I think it is the easiest solution.
Before usage, test it with your compiler carefully.
回答6:
Any of the techniques listed here should work and when C++0x becomes available you will be able to use the built-in static_assert keyword.
回答7:
If you have Boost then using BOOST_STATIC_ASSERT is the way to go. If you're using C or don't want to get Boost
here's my c_assert.h file that defines (and explains the workings of) a few macros to handle static assertions.
It's a bit more convoluted that it should be because in ANSI C code you need 2 different macros - one that can work in the area where you have declarations and one that can work in the area where normal statements go. There is a also a bit of work that goes into making the macro work at global scope or in block scope and a bunch of gunk to ensure that there are no name collisions.
STATIC_ASSERT() can be used in the variable declaration block or global scope.
STATIC_ASSERT_EX() can be among regular statements.
For C++ code (or C99 code that allow declarations mixed with statements) STATIC_ASSERT() will work anywhere.
/*
Define macros to allow compile-time assertions.
If the expression is false, an error something like
test.c(9) : error XXXXX: negative subscript
will be issued (the exact error and its format is dependent
on the compiler).
The techique used for C is to declare an extern (which can be used in
file or block scope) array with a size of 1 if the expr is TRUE and
a size of -1 if the expr is false (which will result in a compiler error).
A counter or line number is appended to the name to help make it unique.
Note that this is not a foolproof technique, but compilers are
supposed to accept multiple identical extern declarations anyway.
This technique doesn't work in all cases for C++ because extern declarations
are not permitted inside classes. To get a CPP_ASSERT(), there is an
implementation of something similar to Boost's BOOST_STATIC_ASSERT(). Boost's
approach uses template specialization; when expr evaluates to 1, a typedef
for the type
::interslice::StaticAssert_test< sizeof( ::interslice::StaticAssert_failed<true>) >
which boils down to
::interslice::StaticAssert_test< 1>
which boils down to
struct StaticAssert_test
is declared. If expr is 0, the compiler will be unable to find a specialization for
::interslice::StaticAssert_failed<false>.
STATIC_ASSERT() or C_ASSERT should work in either C or C++ code (and they do the same thing)
CPP_ASSERT is defined only for C++ code.
Since declarations can only occur at file scope or at the start of a block in
standard C, the C_ASSERT() or STATIC_ASSERT() macros will only work there. For situations
where you want to perform compile-time asserts elsewhere, use C_ASSERT_EX() or
STATIC_ASSERT_X() which wrap an enum declaration inside it's own block.
*/
#ifndef C_ASSERT_H_3803b949_b422_4377_8713_ce606f29d546
#define C_ASSERT_H_3803b949_b422_4377_8713_ce606f29d546
/* first some utility macros to paste a line number or counter to the end of an identifier
* this will let us have some chance of generating names that are unique
* there may be problems if a static assert ends up on the same line number in different headers
* to avoid that problem in C++ use namespaces
*/
#if !defined( PASTE)
#define PASTE2( x, y) x##y
#define PASTE( x, y) PASTE2( x, y)
#endif /* PASTE */
#if !defined( PASTE_LINE)
#define PASTE_LINE( x) PASTE( x, __LINE__)
#endif /* PASTE_LINE */
#if!defined( PASTE_COUNTER)
#if (_MSC_VER >= 1300) /* __COUNTER__ introduced in VS 7 (VS.NET 2002) */
#define PASTE_COUNTER( x) PASTE( x, __COUNTER__) /* __COUNTER__ is a an _MSC_VER >= 1300 non-Ansi extension */
#else
#define PASTE_COUNTER( x) PASTE( x, __LINE__) /* since there's no __COUNTER__ use __LINE__ as a more or less reasonable substitute */
#endif
#endif /* PASTE_COUNTER */
#if __cplusplus
extern "C++" { // required in case we're included inside an extern "C" block
namespace interslice {
template<bool b> struct StaticAssert_failed;
template<> struct StaticAssert_failed<true> { enum {val = 1 }; };
template<int x> struct StaticAssert_test { };
}
}
#define CPP_ASSERT( expr) typedef ::interslice::StaticAssert_test< sizeof( ::interslice::StaticAssert_failed< (bool) (expr) >) > PASTE_COUNTER( IntersliceStaticAssertType_)
#define STATIC_ASSERT( expr) CPP_ASSERT( expr)
#define STATIC_ASSERT_EX( expr) CPP_ASSERT( expr)
#else
#define C_ASSERT_STORAGE_CLASS extern /* change to typedef might be needed for some compilers? */
#define C_ASSERT_GUID 4964f7ac50fa4661a1377e4c17509495 /* used to make sure our extern name doesn't collide with something else */
#define STATIC_ASSERT( expr) C_ASSERT_STORAGE_CLASS char PASTE( PASTE( c_assert_, C_ASSERT_GUID), [(expr) ? 1 : -1])
#define STATIC_ASSERT_EX(expr) do { enum { c_assert__ = 1/((expr) ? 1 : 0) }; } while (0)
#endif /* __cplusplus */
#if !defined( C_ASSERT) /* C_ASSERT() might be defined by winnt.h */
#define C_ASSERT( expr) STATIC_ASSERT( expr)
#endif /* !defined( C_ASSERT) */
#define C_ASSERT_EX( expr) STATIC_ASSERT_EX( expr)
#ifdef TEST_IMPLEMENTATION
C_ASSERT( 1 < 2);
C_ASSERT( 1 < 2);
int main( )
{
C_ASSERT( 1 < 2);
C_ASSERT( 1 < 2);
int x;
x = 1 + 4;
C_ASSERT_EX( 1 < 2);
C_ASSERT_EX( 1 < 2);
return( 0);
}
#endif /* TEST_IMPLEMENTATION */
#endif /* C_ASSERT_H_3803b949_b422_4377_8713_ce606f29d546 */
回答8:
Try:
#define STATIC_ASSERT(x, error) \
do { \
static const char error[(x)?1:-1];\
} while(0)
Then you can write:
STATIC_ASSERT(a == b, a_not_equal_to_b);
Which may give you a better error message (depending on your compiler).
回答9:
The common, portable option is
#if 5 != (state1|mode1)
# error "aaugh!"
#endif
but it doesn't work in this case, because they're C constants and not #defines.
You can see the Linux kernel's BUILD_BUG_ON macro for something that handles your case:
#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
When condition is true, this becomes ((void)sizeof(char[-1])), which is illegal and should fail at compile time, and otherwise it becomes ((void)sizeof(char[1])), which is just fine.
回答10:
Ensure you compile with a sufficiently recent compiler (e.g. gcc -std=c11).
Then your statement is simply:
_Static_assert(state1|mode1 == 5, "Unexpected change of bitflags");
来源：https://stackoverflow.com/questions/174356/ways-to-assert-expressions-at-build-time-in-c