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Concept
Variadic macro in the C preprocessor
Summary
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. Support for variadic macros with no arguments was added in C++20 and will be added in C23.
The declaration syntax is similar to that of variadic functions: a sequence of three full stops "" is used to indicate that one or more arguments must be passed. During macro expansion each occurrence of the special identifier in the macro replacement list is replaced by the passed arguments.
Additionally, regular macro arguments may be listed before the ..., but regular arguments may not be listed after the ....
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.
Both the C99 and C++11 standards require at least one argument, but since C++20 this limitation has been lifted through the functional macro. The macro is replaced by its argument when arguments are present, and omitted otherwise. Common compilers also permit passing zero arguments before this addition, however.
The C preprocessor rules prevent macro names in the argument of from expanding recursively. It is possible to work around this limitation up to an arbitrary fixed number of recursive expansions, however.
Several compilers support variable-argument macros when compiling C and C++ code: the GNU Compiler Collection 3.0, Clang (all versions), Visual Studio 2005, C++Builder 2006, and Oracle Solaris Studio (formerly Sun Studio) Forte Developer 6 update 2 (C++ version 5.3). GCC also supports such macros when compiling Objective-C.
Support for the macro to support zero arguments has been added in GNU Compiler Collection 8, Clang 6, and Visual Studio 2019.
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