Imagine I have an X Macro for a list of items defined something like this:
#define X_MACRO(FN) \
FN(foo) \
FN(bar) \
FN(zip)
This works great and I can call it to generate the same code templatized for each element, like:
#define xstr(s) str(s)
#define str(s) #s
#define PRINT_X(E) void print_ ## E () { std::cout << str(E); };
X_MACRO(PRINT_X)
This generates functions like void print_foo() { std::cout << "foo"; };
for each of the X_MACRO elements. So far, so good.
Now, however, I want the list of X Macro elements to be conditional on a pre-processor macro. For example the zip
element should only be included in the X Macro if USE_ZIP
is defined. Of course, I can't put an #ifdef
inside the X Macro, like:
#define X_MACRO(FN) \
FN(foo) \
FN(bar) \
#ifdef USE_ZIP
FN(zip)
#endif
I could instead write the list twice, once with zip
and once without, based on the USE_ZIP
like so:
#ifdef USE_ZIP
#define X_MACRO(FN) \
FN(foo) \
FN(bar) \
FN(zip)
#else
#define X_MACRO(FN) \
FN(foo) \
FN(bar)
#endif
... but this violates DRY and more importantly it rapidly spirals out of control if you need to conditionally include other elements, which would require a list for each possible combination of USE_*
macros.
How can I do this in a reasonable way?
One way to do this is to split things in a base style and call it from a super macro (I don't know if these have special names):
#define X_MACRO_BASE(fn) \
fn(foo) \
fn(bar) \
#if USE_ZIP
#define X_MACRO(fn) \
X_MACRO_BASE(fn) \
fn(zip)
#else
#define X_MACRO(fn) \
X_MACRO_BASE(fn)
#endif
It's not perfect, but it still might be useful :-)
Another neat trick is to have a simple conditional macro (say if USE_ZIP
was 0
or 1
):
#define IF(cond, foo) IF_IMPL(cond, foo)
#define IF_IMPL(cond, foo) IF_ ## cond (foo)
#define IF_0(foo)
#define IF_1(foo) foo
Then you can say:
#define X_MACRO(fn) \
fn(foo) \
fn(bar) \
IF(USE_ZIP, fn(zip))
How about:
#ifdef USE_ZIP
# define IF_ZIP(arg) arg
#else
# define IF_ZIP(arg)
#endif
#define X_MACRO(MAC) \
MAC(foo) \
MAC(bar) \
IF_ZIP(MAC(zip))
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