How do you perform macro expansion within #ifdef?

Question

I have some fairly generic code which uses preprocessor macros to add a certain prefix onto other macros. This is a much simplified example of what happens:

#define MY_VAR(x) prefix_##x

"prefix_" is actually defined elsewhere, so it will be different each time the file is included. It works well, but now I have some code I would like to skip if one of the tokens doesn't exist, but this doesn't work:

#if defined MY_VAR(hello)

What I want it to expand to is this:

#ifdef prefix_hello

But I can't figure out how. I need to use the MY_VAR() macro to do the expansion, so I can't just hardcode the name. (It's actually for some testing code, the same code gets included with a different prefix each time to test a bunch of classes, and I want to skip a couple of tests for a handful of the classes.)

Is this possible with the C++ preprocessor?

Update:

Here is some semi-compilable code to demonstrate the problem further: (to avoid squishing it into the comments below)

#define PREFIX hello

#define DO_COMBINE(p, x)  p ## _ ## x
#define COMBINE(p, x)     DO_COMBINE(p, x)
#define MY_VAR(x)         COMBINE(PREFIX, x)

// MY_VAR(test) should evaluate to hello_test

#define hello_test "blah blah"

// This doesn't work
#ifdef MY_VAR(test)
  printf("%s\n", MY_VAR(test));
#endif

Answer 1

Is there more to your program than this question describes? The directive

#define MY_VAR(x) prefix_##x

defines exactly one preprocessor identifier. The call

blah ^&* blah MY_VAR(hello) bleh <>? bleh

simply goes in one end of the preprocessor and comes out the other without defining anything.

Without some other magic happening, you can't expect the preprocessor to remember what arguments have been passed into what macros over the course of the preceding source code.

You can do something like

#define prefix_test 1
#if MY_VAR(test) == 1
#undef prefix_test // optional, "be clean"
...
#endif
#undef prefix_test

to query whether the prefix currently has the particular value prefix_. (Undefined identifiers are replaced with zero.)

Answer 2

I think you've got to the level where the preprocessor won't cut it any more; it's really quite simple-minded. Have you considered using templates instead? (Assuming that they're meaningful for your problem, of course.)

Answer 3

This is in response to your updated question. For each possible hello_test, after any possibility of #define hello_test "blah blah", add the lines:

#ifndef hello_test
#define hello_test (char*)0
#endif

Then change your test to:

if (MY_VAR(test))
    printf("%s\n", MY_VAR(test));

Or, as an alternative, before all the #define ..."blah blah"'s, add the line(s):

static const char * const MY_VAR(test);

for all possible values of MY_VAR and test. This will avoid hard-coding "hello_test". (The second const here removes the gcc warning: ‘hello_test’ defined but not used.)