I'm working on some C code that does lot's of error reporting and logging when a DEBUG flag is set, which sometimes produces unused variable warnings when compiling with the DEBUG flag not set.
#ifdef DEBUG
#define CHECK(expr) foo(expr)
#else
#define CHECK(expr)
#endif /* DEBUG */
int x = bar(a, b, c); /* bar has to be called for both DEBUG begin defined and undefined */
CHECK(x == SOME_VALUE); /* Produces an "unused variable" warning if DEBUG is undefined
Edit: Just a little reminder (not sure if it is of any consequence): the argument for the CHECK macro is an expression, not a single variable.
For this pattern, what is the best way to get rid of the unused variable warning?
What I tried/though of:
#ifdef DEBUG
int x = bar(a, b, c);
#else
bar(a, b, c);
#endif
CHECK(x == SOME_VALUE);
and then, to avoid writing the call to bar (which is more complicated in the actual call) twice:
#ifdef DEBUG
int x =
#endif
bar(a, b, c);
CHECK(x == SOME_VALUE);
However, I feel like this is not exactly a clean and readable solution. Is there a better way? Note that for performance reasons the CHECK(expr) macro should not produce any code if DEBUG is undefined (EDIT: and thus, expr should not be evaluated).
Is there a more elegant way than the one I outlined above?
#ifdef DEBUG
#define CHECK(x) x
#else
#define CHECK(x) ((void)sizeof((void)(x),0))
#endif
I think this addresses all of the possible issues :
sizeof ensures that the expression is not evaluated at all, so its side-effects don't happen. That is to be consistent with the usual behaviour of debug-only constructs, such as assert.((x), 0) uses the comma operator to swallow the actual type of (x). This is to prevent VLAs from triggering evaluation.(void) explicitly ignores the result of (x) and sizeof so no "unused value" warning appears.