Templating in C using the Preprocessor - c

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?
This is just considered "bad style"
It's so obvious that nobody would write an article about it
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;
}