#if inside #define?

Question

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);

An `#ifdef` is just a shorter way of saying `#if defined(...)`, so I would say it's not possible. — Some programmer dude, Dec 08 '11 at 08:12

score 6 · Accepted Answer · answered Dec 08 '11 at 08:18

6

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)

answered Dec 08 '11 at 08:18

ronag

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This! Generally, many larger #define's can be reduced to a template and small defines. You'll thank me when having to debug it. – peterchen Dec 08 '11 at 08:26
This is a good way, thanx. It never occured to me i could use smaller template snippetes without changing the whole #define structures in the program :) – Frank Dec 08 '11 at 08:28

score 2 · Answer 2 · answered Dec 08 '11 at 08:20

2

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.

answered Dec 08 '11 at 08:20

DipSwitch

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score 0 · Answer 3 · answered Dec 08 '11 at 08:16

0

Why you dont write it like this:

#if (type == double)
    #define MK_GET  some code
#else
    #define MK_GET  same code with changes
#endif

answered Dec 08 '11 at 08:16

Gil.I

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1

Because then you can only have one define per module and you cannot generate code for all types in one module... – DipSwitch Dec 08 '11 at 08:20

score 0 · Answer 4 · answered Dec 08 '11 at 08:17

0

#if cannot be nested inside #define. Why you want to avoid templates when that is the better choice. They are safe and "compilable" (not preprocessed).

answered Dec 08 '11 at 08:17

iammilind

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Yes ofcrouse. But i dont want to make too big changes to running production code for a small fix. – Frank Dec 08 '11 at 08:23
They actually can...you just have to run the preprocessor more than once (in fact, once for each nesting level). It's a stupid thing to do, but it can be done. See, for instance, the vik2 entry in the international obfuscated c code contest from 2004 (http://www.ioccc.org/years.html#2004) and it's associated Makefile – Daisy Sophia Hollman Jan 26 '12 at 14:03

#if inside #define?

4 Answers4

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