我有以下宏（跟随：When writing a macro in C, how do I find the type and printf specifier for an argument?）：
#define mu_format_specifier(expression) _Generic((expression), unsigned long: "%lu", int: "%i")
#define mu_assert_equal(actual, expected) do { \
if (actual != expected) { \
char *message = malloc(MAX_ERROR_MESSAGE_LENGTH); \
if (message == NULL) { printf("malloc failed"); exit(1); } \
snprintf(message, MAX_ERROR_MESSAGE_LENGTH, \
"required: %s != %s, reality: %s == " mu_format_specifier(actual), \
#actual, #expected, #actual, actual); \
return message; \
} \
} while (0)
问题是mu_format_specifier似乎解析为
char *，而不是简单地将"%lu"替换为"required: %s != %s, reality: %s == " mu_format_specifier(actual),，我想产生"required: %s != %s, reality: %s == " "%lu"，C会将其理解为一个文字字符串。我可以看到两个可怕的解决方法：
为每种类型制作一个mu_assert_equal版本，并使用
_Generic调用正确的版本。snprintf格式化字符串，但是如果格式化字符串必须为
const，我可能会遇到问题。有更好的选择吗？
（问题是
_Generic实际上是对表达式求值，而不是简单地替换源字符-即"%lu"成为它自己的文字字符串，而不仅仅是产生与"required: %s != %s, reality: %s == "统一的源字符。） 最佳答案
我成功了：
#define mu_assert_equal(actual, expected) do { \
if (actual != expected) { \
char *message = malloc(MAX_ERROR_MESSAGE_LENGTH); \
if (message == NULL) { printf("malloc failed"); exit(1); } \
snprintf(message, MAX_ERROR_MESSAGE_LENGTH, _Generic( \
(actual), \
unsigned long: "required: %s != %s, reality: %s == %lu", \
int: "required: %s != %s, reality: %s == %i" \
), \
#actual, #expected, #actual, actual); \
return message; \
} \
} while (0)
解决方案是使整个格式化字符串成为
_Generic的输出表达式。从测试代码：
mu_assert_equal(bytes_parsed, 1);
我得到输出：
required: bytes_parsed != 1, reality: bytes_parsed == 0