{{Short description|Macro taking a varying number of arguments}} 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.<ref>{{Cite web |last=Nelson |first=Clark |date=16 July 2004 |title=Working draft changes for C99 preprocessor synchronization |url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1653.htm |access-date=2025-10-30 |website=www.open-std.org}}</ref> Support for variadic macros with no arguments was added in C++20 and C23.<ref>{{cite web|url=https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0306r3.html|title=Comma omission and comma deletion|date=June 18, 2017|access-date=December 24, 2022}}</ref><ref name=N3033/><!-- The first source is the C++ version and the second the C version. They are not duplicates. -->
== C/C++ == {{See also|C preprocessor|C syntax|C++ syntax}} === Declaration syntax === The declaration syntax is similar to that of variadic functions: a sequence of three full stops "<code>...</code>" is used to indicate that one or more arguments must be passed. During macro expansion each occurrence of the special identifier <code>__VA_ARGS__</code> in the macro replacement list is replaced by the passed arguments.
Additionally, regular macro arguments may be listed before the <code>...</code>,<ref name="gcc" /> but regular arguments may not be listed after the <code>...</code>.
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.<ref>{{cite newsgroup |author= Laurent Deniau |title= __VA_NARG__ |date= 2006-01-16 |newsgroup= comp.std.c |message-id= dqgm2f$ije$1@sunnews.cern.ch |url= http://groups.google.com/group/comp.std.c/browse_thread/thread/77ee8c8f92e4a3fb/346fc464319b1ee5 }}</ref>
Both the C99 and C++11 standards require at least one argument, but since C++20 and C23 this limitation has been lifted through the <code>__VA_OPT__</code> functional macro. The <code>__VA_OPT__</code> macro is replaced by its argument when arguments are present, and omitted otherwise. Common compilers also permit passing zero arguments before this addition, however.<ref name="gcc">[https://gcc.gnu.org/onlinedocs/gcc/Variadic-Macros.html Variadic Macros – Using the GNU Compiler Collection (GCC)]</ref><ref name="msvc">[https://msdn.microsoft.com/en-us/library/ms177415(v=vs.140).aspx Variadic Macros (C++)]</ref>
The C preprocessor rules prevent macro names in the argument of <code>__VA_OPT__</code> from expanding recursively. It is possible to work around this limitation up to an arbitrary fixed number of recursive expansions, however.<ref name="recursive-vaopt">[https://www.scs.stanford.edu/~dm/blog/va-opt.html Recursive macros with C++20 __VA_OPT__]</ref>
=== Support === Several compilers support variable-argument macros when compiling C and C++ code: the GNU Compiler Collection 3.0,<ref name="gcc" /> Clang (all versions),<ref>Clang source code change that mentions __VA_ARGS__ support (2006-07-29), note that Clang was open-sourced in 2007. http://llvm.org/viewvc/llvm-project?view=revision&revision=38770{{Dead link|date=July 2025 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Visual Studio 2005,<ref name="msvc" /> C++Builder 2006, and Oracle Solaris Studio (formerly Sun Studio) Forte Developer 6 update 2 (C++ version 5.3).<ref>Sun Studio feature comparison – http://developers.sun.com/sunstudio/support/CCcompare.html</ref> GCC also supports such macros when compiling Objective-C.
Support for the <code>__VA_OPT__</code> macro to support zero arguments has been added in GNU Compiler Collection 8,<ref>{{cite web|url=https://gcc.gnu.org/projects/cxx-status.html#cxx2a|title=C++2a Support in GCC|access-date=June 14, 2018}}</ref> Clang 6,<ref>{{cite web|url=https://clang.llvm.org/cxx_status.html|title=C++ Support in Clang|access-date=June 14, 2018}}</ref> and Visual Studio 2019.<ref>{{cite web|url=https://docs.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview|title=MSVC new preprocessor overview|date=September 10, 2020|access-date=December 8, 2020}}</ref>
=== Example === If a <code>printf</code>-like function <code>dbgprintf()</code> were desired, which would take the file and line number from which it was called as arguments, the following solution applies. <!-- Don't change the example back to dprintf; it is a POSIX function: http://pubs.opengroup.org/onlinepubs/9699919799/functions/dprintf.html -->
Our implemented function: <syntaxhighlight lang="c"> void realdbgprintf(const char* fileName, int line, const char* fmt, ...); </syntaxhighlight>
Due to limitations of the variadic macro support in C++11 the following straightforward solution can fail and should thus be avoided: <syntaxhighlight lang="cpp"> #define dbgprintf(cformat, ...) realdbgprintf(__FILE__, __LINE__, cformat, __VA_ARGS__) </syntaxhighlight>
The reason is that <syntaxhighlight lang="cpp"> dbgprintf("Hallo") </syntaxhighlight>
gets expanded to <syntaxhighlight lang="cpp"> realdbgprintf(__FILE__, __LINE__, "Hallo", ) </syntaxhighlight>
where the comma before the closing brace will result in a syntax error.
GNU C++ supports a non-portable extension which solves this: <syntaxhighlight lang="c"> #define dbgprintf(cformat, ...) realdbgprintf(__FILE__, __LINE__, cformat, ##__VA_ARGS__) </syntaxhighlight>
C++20 supports the following syntax. <syntaxhighlight lang="cpp"> #define dbgprintf(cformat, ...) realdbgprintf(__FILE__, __LINE__, cformat __VA_OPT__(,) __VA_ARGS__) </syntaxhighlight>
By using the <code>cformat</code> string as part of the variadic arguments we can circumvent the abovementioned incompatibilities. This is tricky but portable. <syntaxhighlight lang="c"> #define dbgprintf(...) realdbgprintf(__FILE__, __LINE__, __VA_ARGS__) </syntaxhighlight>
{{code|dbgprintf()}} could then be called as <syntaxhighlight lang="c"> dbgprintf("Hello, world"); </syntaxhighlight>
which expands to <syntaxhighlight lang="c"> realdbgprintf(__FILE__, __LINE__, "Hello, world"); </syntaxhighlight>
Another example is: <syntaxhighlight lang="c"> dbgprintf("%d + %d = %d", 2, 2, 5); </syntaxhighlight>
which expands to <syntaxhighlight lang="c"> realdbgprintf(__FILE__, __LINE__, "%d + %d = %d", 2, 2, 5); </syntaxhighlight>
Without variadic macros, writing wrappers to <code>printf</code> is not directly possible. The standard workaround is to use the stdargs functionality of C/C++, and have the function call <code>vprintf</code> instead.
=== Trailing comma === There is a portability issue with generating a trailing comma with empty args for variadic macros in C99. Some compilers (e.g., Visual Studio when not using the new standard-conformant preprocessor<ref name="msvc" />) will silently eliminate the trailing comma. Other compilers (e.g.: GCC<ref name="gcc" />) support putting <code>##</code> in front of <code>__VA_ARGS__</code>.
<syntaxhighlight lang="c"> #define MYLOG(FormatLiteral, ...) fprintf(stderr, "%s(%u): " FormatLiteral "\n", __FILE__, __LINE__, __VA_ARGS__) </syntaxhighlight> The following application works <syntaxhighlight lang="c"> MYLOG("Too many balloons %u", 42); </syntaxhighlight> which expands to <syntaxhighlight lang="c"> fprintf(stderr, "%s(%u): " "Too many balloons %u" "\n", __FILE__, __LINE__, 42); </syntaxhighlight> which is equivalent to <syntaxhighlight lang="c"> fprintf(stderr, "%s(%u): Too many balloons %u\n", __FILE__, __LINE__, 42); </syntaxhighlight> But look at this application: <syntaxhighlight lang="c"> MYLOG("Attention!"); </syntaxhighlight> which expands to <syntaxhighlight lang="c"> fprintf(stderr, "%s(%u): " "Attention!" "\n", __FILE__, __LINE__, ); </syntaxhighlight> which generates a syntax error with GCC.
GCC supports the following (non-portable) extension: <syntaxhighlight lang="c"> #define MYLOG(FormatLiteral, ...) fprintf(stderr, "%s(%u): " FormatLiteral "\n", __FILE__, __LINE__, ##__VA_ARGS__) </syntaxhighlight> which removes the trailing comma when <code>__VA_ARGS__</code> is empty.
C23 solves this problem by introducing <code>__VA_OPT__</code> like C++.<ref name=N3033>{{cite web|url=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3033.htm|title=WG14 - N3033 : Comma omission and comma deletion|date=2022-07-20}}</ref>
=== Alternatives === Before the existence of variable-arguments in C99, it was quite common to use doubly nested parentheses to exploit the variable number of arguments that could be supplied to the <code>printf()</code> function:
<syntaxhighlight lang="c"> #define dbgprintf(x) realdbgprintf x </syntaxhighlight> {{code|dbgprintf()}} could then be called as: <syntaxhighlight lang="c"> dbgprintf(("Hello, world %d", 27)); </syntaxhighlight> which expands to: <syntaxhighlight lang="c"> realdbgprintf("Hello, world %d", 27); </syntaxhighlight>
== Rust == {{Main|Rust syntax#Macros}} In Rust, a variadic interface (also known as a variadic macro)<ref>{{Cite web|title=Variadic Interfaces|url=https://doc.rust-lang.org/rust-by-example/macros/variadics.html|publisher=Rust by Example|author=The Rust Team|website=rust-lang.org|date=1 April 2026}}</ref>, allows for taking an arbitrary number of arguments. Rust lacks variadic functions and variadic templates, thus the only way variadic parameters can be accomplished is through said variadic interfaces, which use repetition patterns through <code>macro_rules!</code>. For example, in <syntaxhighlight lang="rust" inline>$( ... ),*</syntaxhighlight>, <code>$()</code> denotes a repeating pattern, <code>,</code> denotes a separator, <code>*</code> denotes zero or more repetitions (a Kleene star, while <code>+</code> which matches one or more would be a Kleene plus).
One such example of implementing a variadic macro is the following, equivalent to <code>vec!</code>: <syntaxhighlight lang="rust"> // Define a macro named "make_vec!" macro_rules! make_vec { // Matches any Rust expression // and repeat the pattern separated by commas, zero or more times ( $( $x:expr ),* ) => { { // Vec must be fully qualified // to prevent resolving to a user-defined Vec let mut temp = ::std::vec::Vec::new(); // Repeat a push to temp once per argument $( temp.push($x); )* // Return temp from the macro temp } }; } </syntaxhighlight>
== References == {{Reflist}}
== See also == * Variadic function * Variadic template
{{CProLang}} {{C++ programming language}}
Category:C (programming language) Category:C++ Category:Rust (programming language)