Thread: Static assert of define

How do you expect the compile time pre-processor to deal with run-time values?

If you want to do this, make it an inline function rather than a macro.

What examples of use in context do you have?

The inline function idea might be worth a try: while inlining isn't guaranteed, if it does happen (and the code does look like a good candidate for inlining), it may be possible that the compiler will do constant propagation and then eliminate the branch that it can determine will never be executed.

Originally Posted by MartinR

The usage is as I mention - the programmer will always use the macro with constants known at compile time. Now the question is if we can verify them inside that #define ?

Usage - like

Code:

if ( IS_BIT_SET(255,8) ) {
  // do something
}

Really limits your options as to what is syntactically valid inside your macro.

Things which can exist as statements have more scope.

Code:

#define STATIC_ASSERT(COND,MSG) typedef char static_assertion_##MSG[(COND)?1:-1]

#define TOGGLE_BIT(x,y) do {          \
  STATIC_ASSERT((y<32),out_of_bound); \
  (x) ^= (1 << (y));                  \
} while ( 0 )

int main ( ) {
  int x = 0;
  TOGGLE_BIT(x,44);
}


$ gcc foo.c
foo.c: In function ‘main’:
foo.c:1:46: error: size of array ‘static_assertion_out_of_bound’ is negative
 #define STATIC_ASSERT(COND,MSG) typedef char static_assertion_##MSG[(COND)?1:-1]
                                              ^
foo.c:4:3: note: in expansion of macro ‘STATIC_ASSERT’
   STATIC_ASSERT((y<32),out_of_bound); \
   ^
foo.c:10:3: note: in expansion of macro ‘TOGGLE_BIT’
   TOGGLE_BIT(x,44);
   ^
foo.c:5:13: warning: left shift count >= width of type [-Wshift-count-overflow]
   (x) ^= (1 << (y));                  \
             ^
foo.c:10:3: note: in expansion of macro ‘TOGGLE_BIT’
   TOGGLE_BIT(x,44);
   ^

But note that gcc already knows how to warn you about these things if the shift is known at compile time.

> But shouldn't compiler complain about redefinition of a typedef?
Well each is scoped by the braces of the do-while, so no.