How can I generate a list via the C preprocessor (cpp)?

问题

I would like to do something like the following:

F_BEGIN

F(f1) {some code}
F(f2) {some code}
...
F(fn) {some code}

F_END

and have it generate the following

int f1() {some code}
int f2() {some code}
...
int fn() {some code}

int (*function_table)(void)[] = { f1, f2, ..., fn };

The functions themselves are easy. What I can't seem to do is to keep track of all of the names until the end for the function_table.

I looked at this question and this question but I couldn't get anything to work for me. Any ideas?

回答1:

The normal way of doing this with the preprocessor is to define all the functions in a macro that takes another macro as an argument, and then use other macros to extract what you want. For your example:

#define FUNCTION_TABLE(F) \
    F(f1, { some code }) \
    F(f2, { some code }) \
    F(f3, { some code }) \
:

    F(f99, { some code }) \
    F(f100, { some code })

#define DEFINE_FUNCTIONS(NAME, CODE)     int NAME() CODE
#define FUNCTION_NAME_LIST(NAME, CODE)   NAME,

FUNCTION_TABLE(DEFINE_FUNCTIONS)
int (*function_table)(void)[] = { FUNCTION_TABLE(FUNCTION_NAME_LIST) };

回答2:

If you have a C99 complying compiler, the preprocessor has variable length argument lists. P99 has a preprocessor P99_FOR that can do "code unrolling" like the one you want to achieve. To stay close to your example

#define MYFUNC(DUMMY, FN, I) int FN(void) { return I; } 
#define GENFUNCS(...)                                          \
P99_FOR(, P99_NARG(__VA_ARGS__), P00_IGN, MYFUNC, __VA_ARGS__) \
int (*function_table)(void)[] = { __VA_ARGS__ }

GENFUNCS(toto, hui, gogo);

would expand to the following (untested)

int toto(void) { return 0; } 
int hui(void) { return 1; }
int gogo(void) { return 2; }
int (*function_table)(void)[] = { toto, hui, gogo };

回答3:

This is sort of abuse of CPP but a common type of abuse. I handle situations like this by defining dummy macros

#define FUNCTIONS \
 foo(a,b,c,d) \
 foo(a,b,c,d) \
 foo(a,b,c,d)

now, 

#define foo(a,b,c,d) \
 a+b ;

FUNCTIONS

#undef foo

later, when you want something different done with the same list

#define foo(a,b,c,d) \
 a: c+d ;

FUNCTIONS

#undef foo

It's a bit ugly and cumbersome, but it works.

回答4:

Boost is a C++ library, but it's Preprocessor module should still be good for use in C. It offers some surprisingly advanced data types and functionality for use in the preprocessor. You could check it out.

来源：https://stackoverflow.com/questions/4904255/how-can-i-generate-a-list-via-the-c-preprocessor-cpp

标签

c-preprocessor