Passing a "value", that was either #defined or #undef, into a macro and check if it was defined?

[arnold_w]

I want to create a macro that is fully executed at compile time (not runtime). If two values are both defined, then I want to check if they are equal. If at least one of them is not defined, then nothing should happen. I have come halfway, the following works great:

#define VALIDATE_ADDR(ADDR1,ADDR2) _Static_assert(ADDR1 == ADDR2, #ADDR1 " is not equal to " #ADDR2);

However, when I try to nest that macro into another macro, then I run into trouble:

#define VALIDATE_ADDR_IF_THEY_BOTH_EXIST(ADDR1,ADDR2) \
#if defined(ADDR1) && defined(ADDR2) \
VALIDATE_ADDR(ADDR1, ADDR2) \
#endif

Can someone please help me so I can do checks like this:

#define MY_ADDR_1 (1)
#define MY_ADDR_2 (2)
#define MY_ADDR_3 (3)
#ifdef MY_ADDR_4
#undef MY_ADDR_4
#endif

VALIDATE_ADDR_IF_THEY_BOTH_EXIST(MY_ADDR_1, MY_ADDR_2); // Compile time error since MY_ADDR_1 != MY_ADDR_2
VALIDATE_ADDR_IF_THEY_BOTH_EXIST(MY_ADDR_3, MY_ADDR_4); // Compiles just fine since MY_ADDR_4 is not defined

[k5054]

AFAIK, that's not possible, since the # token is reserved for stringifying macro parameters within the macro expansion. Also, I see that your definition for VALIDATE_ADDR ends in a semicolon. In general, you do not want to end a macro with a semicolon, since this can lead to, in the best case, a compiler error, and in the worst case, a subtle and hard to find bug.

[Lost User]

Macros are purely compile time. If you want runtime checks then use functions. Macros such as the above tend to be more trouble than they are worth.

[arnold_w]

What do you mean? I've clearly stated I want compile time execution, not runtime execution of this macro. Are you thinking of the _Static_assert function? It is executed at compile time, I think it's a feature built into the gcc compiler.

[arnold_w]

Btw, I don't need the VALIDATE_ADDR macro for anything, it's perfectly fine to replace it with the _Static_assert directly, if that makes anything easier.

[Lost User]

Sorry I misread the question. I have tried a number of variants but cannot get it to work. it would appear that #define statements are not accepted inside a macro definition.