Any workaround for self-referencing macros in C? I want to add a macro
I have a file that uses a macro FLAGS
from include that I have no control over. What is in FLAGS
is incompatible. Sometimes I need to change FLAGS
to add a flag to it. Now I know I can't do it #define FLAGS FLAGS|MY_FLAG
, but I thought that if I was storing FLAGS
in a temporary variable, then I could define and redefine it using the temporary and my flag. For example:
// Assume this next line is what in the include file
#define FLAGS (1|2|4)
// The rest of this is source, assume compile with -DMOD
#ifdef MOD
#define TEMP (FLAGS|8)
#undef FLAGS
#define FLAGS TEMP
#endif
int main()
{
printf("0x%x\n", FLAGS);
}
And if the MOD
error is defined equals error: 'FLAGS' was not declared in this scope
. I know that I can change all the actual C code it uses FLAGS
instead of using it FLAGS|MY_FLAG
, but I was hoping to change the macro and not all the code.
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You can do something logically equivalent #define FLAGS FLAGS|MY_FLAG
if you define the macro to be mutable using Boost's "evaluated slots" :
#include <boost/preprocessor/slot/slot.hpp>
// define FLAGS as a modifiable macro and create a setter for it
#define FLAGS BOOST_PP_SLOT(1)
#define UPDATE_FLAGS BOOST_PP_ASSIGN_SLOT(1)
int main(void) {
// set the initial value of FLAGS
#define BOOST_PP_VALUE (1|2|4)
#include UPDATE_FLAGS
printf("0x%x\n", FLAGS); // 0x7
// update FLAGS with a new value using the old one
#define BOOST_PP_VALUE (FLAGS|8)
#include UPDATE_FLAGS
printf("0x%x\n", FLAGS); // 0xf
}
Even though it is sorcery, it is completely standard C, no extensions. Only works for integers.
(It works by taking advantage of something important: macros are not just expanded into code, but must also be expanded to determine whether to follow a branch #if
. Since directives are #if
also capable of evaluating integer math, this can expand the actual numeric value and use its to build a new extension for PP_SLOT
that doesn't include a reference to any macro name. It's all hidden behind directives #include UPDATE_FLAGS
.)
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Your only real way to do what you are trying to do is by defining an additional macro
// Assume this next line is what in the include file
#define FLAGS_FOR_A (1|2|4)
#define FLAGS FLAGS_FOR_A
// The rest of this is source, assume compile with -DMOD
#ifdef MOD
#undef FLAGS
#define FLAGS ( FLAGS_FOR_A | 8 )
#endif
int main()
{
printf("0x%x\n", FLAGS);
}
Macros simply perform substitution of plain text computed before execution
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