使用预处理器在C中进行模板化

Question

I'm wondering why I have never seen the following way to implement templates in C before. My idea was to make the Preprocessor to the templating-work.

container.h :

#ifndef TEMPLATE_TYPE
    #error "missing decalaration TEMPLATE_TYPE"
#endif

#define _CONCAT(a, b) a##b
#define _EVALUATOR(a, b) _CONCAT(a, b)
#define MAKE_NAME(a, b) _EVALUATOR(a, b)

typedef struct {
    TEMPLATE_TYPE   data;
} MAKE_NAME(Container_, TEMPLATE_TYPE);

main.c :

#define TEMPLATE_TYPE int
#include "container.h"

int main() {
    Container_int c;
    c.data = 99923;
}

So, what's the case?

1. This is just considered "bad style"
2. It's so obvious that nobody would write an article about it
3. There are a lot of articles, just google man!

I would appreciate comments about this technique when you are not planning to answer with #3.

Answer 1

You can do incredible things (good and evil) with the preprocessor. Whether it's considered bad style or not is a judgement call, and it largely depends on the quality, readability, and maintainability of the code that results. Complicated preprocessor macros are a pain to write, debug, and maintain. However, the best C code is the code you don't write, and macros are great for automatically generating variations on a theme.

Here are some good examples of preprocessor (ab)use:

• The SimpleScalar processor simulator's machine.def file (which is C code, despite the extension)
• The canonical queue.h linked-list headers

The SimpleScalar code uses a pattern like your suggestion, above, where the #include is preceded by a #define that gives the header some direction.

If you're considering serious use of the preprocessor, you should look at the Boost preprocessor library . (Don't be put off by Boost's C++ roots, the preprocessor macros work fine with C.)

Answer 2

Instead of

typedef struct {
    TEMPLATE_TYPE   data;
} MAKE_NAME(Container_, TEMPLATE_TYPE)

you might want to do

#define MAKE_CONTAINER(type) typedef struct MAKE_NAME(Container_, type) { type data; } MAKE_NAME(Container_, type)

in order to be able to do

#include "container.h"
MAKE_CONTAINER(int);
MAKE_CONTAINER(double);

int main() {
    Container_int c; // one way to go
    struct Container_double d; // my preferred way: don't typedef when not needed; let the structs be obvious.

    c.data = 99923;
    d.data = 3.5;
}