How to implement C/C++ variadic logging macro with __FILE__ and __LINE__ info for multiple platforms? [duplicate]

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Standard alternative to GCC's ##__VA_ARGS__ trick? (12 answers)

#define macro for debug printing in C? (15 answers)

Closed 5 years ago.

I want to implement a C/C++ variadic logging macro, which contains __FILE__ and __LINE__ information.

This is my simple implementation:

#include <stdio.h>
#define MYLOG(format, ...) printf("%s:%d " format, __VA_ARGS__)

The only issue is that, this macro doesn't compile when my logging has no parameters, for example:

MYLOG("hello world");

I've read some wikis and blogs, there's a solution for GCC compiler:

#include <stdio.h>
#define MYLOG(format, ...) printf("%s:%d " format, ##__VA_ARGS__)

But is there a more standard way to implement this macro working on GCC/Clang/MSVC compilers?

• Remy Lebeau: What about simply using 2 different macros? #define MYLOG(text) printf("%s:%d %s", __FILE__, __LINE__, text) ... #define MYLOG_FMT(format, ...) printf("%s:%d " format, __FILE__, __LINE__, __VA_ARGS__) Call MYLOG() when you don't have format parameters, and call MYLOG_FMT() when you do.
• linrongbin: @RemyLebeau, It really solve the issue, but I'm just looking for a killer solution.
• Jonathan Leffler: Take a good look at #define macro for debug printing in C — it covers most of the territory you need, and includes passing reference to adding __FILE__ and __LINE__ information to the logging.
• Antti Haapala: stackoverflow.com/a/8673872/918959
• Antti Haapala: Also there is no such language as C/C++. Either something is C or is not, something is C++ or not. A minority of code is in the intersection. What it is that you want?

Answer 1

If your compiler supports C++20 then there is a standard way to solve this problem using __VA_OPT__. __VA_OPT__(,) will expand to a comma if __VA_ARGS__ is not empty. So when __VA_ARGS__ is empty, there's no extra comma and no compilation error.

#define MYLOG(format, ...) printf("%s:%d " format __VA_OPT__(,) __VA_ARGS__)

• linrongbin: It looks __VA_OPT__ is the only standard way.