How to test if preprocessor symbol is #define'd but has no value?

Question

Using C++ preprocessor directives, is it possible to test if a preprocessor symbol has been defined but has no value? Something like that:

#define MYVARIABLE
#if !defined(MYVARIABLE) || #MYVARIABLE == ""
... blablabla ...
#endif

EDIT: The reason why I am doing it is because the project I'm working on is supposed to take a string from the environment through /DMYSTR=$(MYENVSTR), and this string might be empty. I want to make sure that the project fails to compile if user forgot to define this string.

Answer 1

I don't think that this can be done. That being said, I don't see a need for it. When you make a preprocessor #define symbol, you should establish a convention that either you define it as 1 or 0 for use in #if, or you leave it blank.

Answer 2

You can't since the preprocessor can only check for a numeric value. You string compare is not covered by preprocessor syntax.

Answer 3

I want to make sure that the project fails to compile if user forgot to define this string.

While i'd check this in a previous build-step, you can do this at compile-time. Using Boost for brevity:

#define A "a"
#define B
BOOST_STATIC_ASSERT(sizeof(BOOST_STRINGIZE(A)) > 1); // succeeds
BOOST_STATIC_ASSERT(sizeof(BOOST_STRINGIZE(B)) > 1); // fails

Answer 4

Soma macro magic:

#define DO_EXPAND(VAL)  VAL ## 1
#define EXPAND(VAL)     DO_EXPAND(VAL)

#if !defined(MYVARIABLE) || (EXPAND(MYVARIABLE) == 1)

Only here if MYVARIABLE is not defined
OR MYVARIABLE is the empty string

#endif

Note if you define MYVARIABLE on the command line the default value is 1

g++ -DMYVARIABLE <file>

Here the value of MYVARIABLE is 1

g++ -DMYVARIABLE= <file>

Here the value of MYVARIABLE is the empty string

The quoting problem solved:

#define DO_QUOTE(X)        #X
#define QUOTE(X)           DO_QUOTE(X)

#define MY_QUOTED_VAR      QUOTE(MYVARIABLE)

std::string x = MY_QUOTED_VAR;
std::string p = QUOTE(MYVARIABLE);