Counting preprocessor macros

问题:

I have this macro code, which allows me to define both a C enum and a list of the enumerated names as strings using one construct. It prevents me from having to duplicate enumerator names (and possibly introducing errors for large lists).

#define ENUM_DEFINITIONS(F) \
  F(0, Item1) \
  F(5, Item2) \
  F(15, Item3) \
  ...
  F(63, ItemN)

then:

enum Items {
  #define ITEM_ENUM_DEFINE(id, name) name = id,
    ENUM_DEFINITIONS(ITEM_ENUM_DEFINE)
  #undef ITEM_ENUM_DEFINE

which, when expanded, should produce:

enum Items {
  Item1 = 0,
  Item2 = 5,
  Item3 = 15,
  ...
  ItemN = 63,
}

In the implementation file, I have this code:

const char* itemNames[TOTAL_ITEMS];
int iter = 0;

#define ITEM_STRING_DEFINE(id, name) itemNames[iter++] = #name;
  ENUM_DEFINITIONS(ITEM_STRING_DEFINE)
#undef ITEM_STRING_DEFINE

which, when expanded, produces:

itemNames[iter++] = "Item1";
itemNames[iter++] = "Item2";
itemNames[iter++] = "Item3";
...
itemNames[iter++] = "ItemN";

I'd like to know how many enumerator items I've created in this fashion and be able to pass it to compile-time arrays. In the example above, this would be determining that TOTAL_ITEMS = N at compile-time. Is it possible to count macro invocations in this way?

I've seen mention of a non-standard COUNTER macro, similar to the FILE and LINE macros, but I'm hoping there is a more standard way.

Would also be interested in hearing if there is a better way to achieve this without having to use macros.

回答1:

The following should work:

#define ITEM_STRING_DEFINE(id, name) #name, // note trailing comma
const char *itemNames[] = {
  ENUM_DEFINITIONS(ITEM_STRING_DEFINE)
};

#define TOTAL_ITEMS (sizeof itemNames / sizeof itemNames[0])

Edit: Thank you to Raymond Chen for noting we don't have to worry about the unnecessary final comma in the list. (I had been misremenbering the problem for enums with strict C89 compilers, as in Is the last comma in C enum required?.)

回答2:

You can use the same technique to count the invocations.

enum itemscounter  { 
  #define ITEM_ENUM_DEFINE(id, name) name##counter,
    ENUM_DEFINITIONS(ITEM_ENUM_DEFINE) 
  #undef ITEM_ENUM_DEFINE 
 TOTAL_ITEMS
};

回答3:

Would also be interested in hearing if there is a better way to achieve this without having to use macros.

You could always use a scripting language such as ruby or python to generate .c and .h files for you. If you do it well, you can integrate your script into your Makefile.

回答4:

I know this isn't a complete answer. You can create a macro around something like this.

#include <stdio.h>

const char * array[] = {
  "arr1", "arr2", "arr3", "arr4"
};

int main (int argc, char **argv)$
{
  printf("%d\n", sizeof(array)/sizeof(const char *));
}

If you can modify your enum so it has continous elements you can do sth like this (from Boost)

enum { A=0,B,C,D,E,F,N };
const char arr[N]; // can contain a character for each enum value

回答5:

See the suggestions Mu Dynamics 'Enums, Strings and Laziness'; these are at least related to what you're after.

Otherwise, look at the Boost Preprocessor collection (which is usable with the C preprocessor as well as the C++ preprocessor).