#if inside #define?
-
10-03-2021 - |
Pregunta
I'm sitting on some legacy code that generates a lot of code through #defines. Now I know it's not possible to have an #ifdef
inside a #define
, but is an #if
possible? I would like to do add some specialization for a specific type. (without making major changes like using templates instead). The following sample gives me cryptic errors so this is not the way:
#define MK_GET(type) \
type get_ ## type (int index) \
{ \
#if type == double \ <-- what i want to add
specialized code... \
#endif
...
} \
MK_GET(double);
MK_GET(int);
MK_GET(string);
Solución
You could achieve that with templates:
template<typename T>
struct getter
{
T operator()(int index)
{
// general code
}
};
template<>
struct getter<double>
{
T operator()(int index)
{
// specialized code
}
};
#define CAT(a, b) a ## b
#define MK_GET(type) type CAT(get_, type) (int index) getter<type>()(index)
Otros consejos
Why you dont write it like this:
#if (type == double)
#define MK_GET some code
#else
#define MK_GET same code with changes
#endif
#if
cannot be nested inside #define
. Why you want to avoid template
s when that is the better choice. They are safe and "compilable" (not preprocessed).
The pre-processor is one pass process, so you can't place macro defines inside of macro defines. The only way to do this is via the template interface.