A variadic macro is a feature of some computer programming languages, especially the C preprocessor, whereby a macro may be declared to accept a varying number of arguments.
Variable-argument macros were introduced in 1999 in the ISO/IEC 9899:1999 (C99) revision of the C language standard, and in 2011 in ISO/IEC 14882:2011 (C++11) revision of the C++ language standard.[1]
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Declaration syntax [link]
The declaration syntax is similar to that of variadic functions: an ellipsis "..." is used to indicate that one or more arguments must be passed. Common compilers also permit passing zero arguments to such a macro, however.[2][3] During macro expansion each occurrence of the special identifier __VA_ARGS__ in the macro replacement list is replaced by the passed arguments.
No means is provided to access individual arguments in the variable argument list, nor to find out how many were passed. However, macros can be written to count the number of arguments that have been passed.[4]
Support [link]
Several compilers support variable-argument macros when compiling C and C++ code: the GNU Compiler Collection 3.0,[2] Visual Studio 2005,[3] C++Builder 2006, and Oracle Solaris Studio (formerly Sun Studio) Forte Developer 6 update 2 (C++ version 5.3).[5] GCC also supports such macros when compiling Objective-C.
Example [link]
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:
<source lang="c"> void realdbgprintf (char const *file, int line, const char *format, ...); // note that format is passed as part of __VA_ARGS__ to ensure it is not empty
- define dbgprintf(...) realdbgprintf(__FILE__, __LINE__, __VA_ARGS__)
</source> dbgprintf() could then be called as: <source lang="c"> dbgprintf("Hello, world"); </source> which expands to: <source lang="c"> realdbgprintf(__FILE__, __LINE__, "Hello, world"); </source> or: <source lang="c"> dbgprintf("%d + %d = %d", 2, 2, 5); </source> which expands to: <source lang="c"> realdbgprintf(__FILE__, __LINE__, "%d + %d = %d", 2, 2, 5); </source> 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.
References [link]
- ^ Working draft changes for C99 preprocessor synchronization - https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1653.htm
- ^ a b https://gcc.gnu.org/onlinedocs/gcc/Variadic-Macros.html
- ^ a b https://msdn.microsoft.com/en-us/library/ms177415(v=vs.80).aspx
- ^ Laurent Deniau (2006-01-16). "__VA_NARG__". comp.std.c. (Web link).
- ^ Sun Studio feature comparison - https://developers.sun.com/sunstudio/support/CCcompare.html
