C++ 可变参数宏到可变参数函数

Question

Implementing an assertion macro I stumbled upon a problem on Xcode clang.

I use the following code:

void Log(LogLevel logLevel, const std::string& message);

std::string FormatString(const char* format, ...);

#define LOG(Level, Format, ...)                                     \
{                                                                   \
    std::string _ac_err_str_ = FormatString(Format, ##__VA_ARGS__); \
    Log(Level, _ac_err_str_);                                       \
}

#define checkf(expr, Format, ...)   { if(!(expr)) { LOG(LL_ErrorMessage, TXT("Log in file ") __FILE__ TXT(" at line %d: ") TXT(#expr) TXT(". ") Format, __LINE__, __VA_ARGS__); } }

Everything works fine with MSVC but on Clang I get the following error: Expected expression

I've tracked down the problem on this line:

std::string _ac_err_str_ = FormatString(Format, ##__VA_ARGS__);

More specifically the problem is on the ##__VA_ARGS__ part because if I remove it everything compiles.

Also, if I pass a third parameter to the macro it compiles, eg:

checkf(false, "Error message");       // Won't compile
checkf(false, "Error %s", "message"); // Will compile

It seems like there's an error on passing the macro's variadic arguments to FormatString , but I can't find out the reason or how to fix this.

Answer 1

Apparently on Clang I need to change my checkf macro like this

#define checkf(expr, Format, ...)   { if(!(expr)) { LOG(LL_ErrorMessage, TXT("Log in file ") __FILE__ TXT(" at line %d: ") TXT(#expr) TXT(". ") Format, __LINE__, ##__VA_ARGS__); } }

Notice that I've used ##__VA_ARGS__ instead of just __VA_ARGS__ .

It compiles with both MSVC and Clang as well.