Can macros accept types? - objective-c

Question

Unless my understanding is incorrect, the following macro
int i; // for loop
const char* ctype; // proprietary type string
void** pool = malloc(sizeof(void*) * (nexpected - 1));
size_t poolc = 0;
#define SET(type, fn) type* v = (pool[poolc++] = malloc(sizeof(type))); \
*v = (type) fn(L, i)
#define CHECK(chr, type, fn) case chr: \
SET(type, fn); \
break
switch (ctype[0]) {
CHECK('c', char, lua_tonumber);
}
should expand to
int i; // for loop
const char* ctype; // proprietary type string
void** pool = malloc(sizeof(void*) * (nexpected - 1));
size_t poolc = 0;
switch (ctype[0]) {
case 'c':
char* v = (pool[poolc++] = malloc(sizeof(char)));
*v = (char) lua_tonumber(L, i);
break;
}
but upon compilation, I get:
src/lua/snip.m:185:16: error: expected expression
CHECK('c', char, lua_tonumber);
^
src/lua/snip.m:181:9: note: expanded from macro 'CHECK'
SET(type, fn); \
^
src/lua/snip.m:178:23: note: expanded from macro 'SET'
#define SET(type, fn) type* v = (pool[poolc++] = malloc(sizeof(type))); \
^
src/lua/snip.m:185:5: error: use of undeclared identifier 'v'
CHECK('c', char, lua_tonumber);
^
src/lua/snip.m:181:5: note: expanded from macro 'CHECK'
SET(type, fn); \
^
src/lua/snip.m:179:6: note: expanded from macro 'SET'
*v = (type) fn(L, i)
^
2 errors generated.
What is going on here? Isn't the preprocessor a literal text replacement engine? Why is it trying to evaluate expressions?
Keep in mind while this looks like straight C, this is actually clang Objective C (note the .m) under the C11 standard. Not sure if that makes any difference.
I'm a loss at how to continue without expanding the code for each entry.

Answer 1

Your understanding is correct! But you're running into a quirk of the C language. A label, including a case label, must be followed by an expression, not a variable declaration.
You can work around this by inserting a null statement (e.g, 0;) after the case, or by enclosing the case body in a set of braces. A practical way of doing this might be by redefining CHECK as:
#define CHECK(chr, type, fn) \
case chr: { SET(type,fn); } break;