Variadic Macro - Example

Example

If a printf-like function dbgprintf were desired, which would take the file and line number from which it was called as arguments, the following macro might be used:

// Our implemented function void realdbgprintf (const* char SourceFilename, int SourceLineno, const char *CFormatString, ...); // Due to a design bug of the variadic macro in C++, the following is more readable, but should be avoided. // Reason is, that dbgprintf("Hallo") will be expanded to realdbgprintf (__FILE__, __LINE__, "Hallo", ) // The comma before the closing brace will result in a syntax error ... //#define dbgprintf(cformat, ...) realdbgprintf (__FILE__, __LINE__, cformat, __VA_ARGS__) // GNU C++ supports the non portable extention: //#define dbgprintf(cformat, ...) realdbgprintf (__FILE__, __LINE__, cformat, ##__VA_ARGS__) // By using the cformat string as part of the variadic arguments we can avoid this. // This is tricky, but portable solution. #define dbgprintf(...) realdbgprintf (__FILE__, __LINE__, __VA_ARGS__)

dbgprintf could then be called as:

dbgprintf ("Hello, world");

which expands to:

realdbgprintf (__FILE__, __LINE__, "Hello, world");

or:

dbgprintf("%d + %d = %d", 2, 2, 5);

which expands to:

realdbgprintf(__FILE__, __LINE__, "%d + %d = %d", 2, 2, 5);

Without variadic macros, writing wrappers to printf is not directly possible. The standard workaround is to use the stdargs functionality of C/C++, and have the function call vprintf instead.