宏可以接受类型吗？

Question

除非我理解有误，下面的宏

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);
}

应该扩展到

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;
}

但编译后，我得到：

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.

这是怎么回事？预处理器不是文字文本替换引擎吗？为什么要尝试计算表达式？

请记住，虽然这看起来像纯 C，但实际上是 C11 标准下的 clang Objective C（注意 .m）。不确定这是否有任何区别。

我不知道如何在不扩展每个条目的代码的情况下继续。

Answer 1

您的理解是正确的！但是您运行陷入了 C 语言的怪癖。 A label, including a case label, must be followed by an expression, not a variable declaration.

您可以通过在 case 之后插入空语句（例如 0;）或将 case 主体括在一组大括号中来解决此问题。一种实用的方法是将 CHECK 重新定义为：

#define CHECK(chr, type, fn) \
    case chr: { SET(type,fn); } break;

宏可以接受类型吗？

Can macros accept types?

c

objective-c

c-preprocessor