Question

Just as the title says. I want to use a preprocessor macro in the text of an #error statement:

#define SOME_MACRO 1

#if SOME_MACRO != 0
    #error "SOME_MACRO was not 0; it was [value of SOME_MACRO]"
#endif

In this example I want the preprocessor to resolve [value of SOME_MACRO] to the actual value of SOME_MACRO which in this case is 1. This should happen before the preprocessor, compiler or whatever processes #error prints the error output
Is there a way to do that or is this just not possible?

I don’t want to know if there is an ISO C++ standard way to do that, because afaik the preprocessor directive #error is not stated in any ISO C++ standard. However, I know GCC and Visual C++ support #error. But my question is not specific to those compilers, I’m just curious if any C/C++ compiler/preprocessor can do that.

I tried to search for that topic but without any luck.

Answer 1

For completeness the C++0x way I suggested (using the same trick as Kirill):

#define STRING2(x) #x
#define STRING(x) STRING2(x)

#define EXPECT(v,a) static_assert((v)==(a), "Expecting " #v "==" STRING(a) " [" #v ": "  STRING(v) "]")


#define VALUE 1

EXPECT(VALUE, 0);

Gives:

g++ -Wall -Wextra -std=c++0x test.cc                     
test.cc:9: error: static assertion failed: "Expecting VALUE==0 [VALUE: 1]"