Allows a function to accept any number of extra arguments.

A function is a variadic if the last parameter of itsparameter list is an ellipsis (...).

// the function declared as followsint printx(constchar* fmt, ...);int printx(constchar* fmt...);// same as above, but deprecated since C++26 // may be called with one or more arguments:printx("hello world");printx("a=%d b=%d", a, b); int printy(...,constchar* fmt);// error: ... can only be the last parameterint printz(...);// valid, but the arguments cannot be accessed portably

[edit]Default argument promotions

When a variadic function is called, after lvalue-to-rvalue, array-to-pointer, and function-to-pointerconversions, each argument that is a part of the variable argument list undergoes additional conversions known asdefault argument promotions:

• float arguments are converted todouble as infloating-point promotion.
• bool,char,short, and unscoped enumerations are converted toint or wider integer types as inintegral promotion.

Non-POD class types(until C++11)Scoped enumerations and class types with an eligible non-trivial copy constructor, an eligible non-trivial move constructor, or a non-trivial destructor(since C++11) are conditionally-supported in potentially-evaluated calls with implementation-defined semantics (these types are always supported inunevaluated calls).

Because variadic parameters have the lowest rank for the purpose ofoverload resolution, they are commonly used as the catch-all fallbacks inSFINAE.

Within the body of a function that uses variadic arguments, the values of these arguments may be accessed using the<cstdarg> library facilities:

The behavior of theva_start macro is undefined if the last parameter before the ellipsis has reference type, or has type that is notcompatible with the type that results from default argument promotions.

[edit]Alternatives

[edit]Notes

In the C programming language until C23, at least one named parameter must appear before the ellipsis parameter, soR printz(...); is not valid until C23. In C++, this form is allowed even though the arguments passed to such function are not accessible, and is commonly used as the fallback overload inSFINAE, exploiting the lowest priority of the ellipsis conversion inoverload resolution.

This syntax for variadic arguments was introduced in 1983 C++ without the comma before the ellipsis. When C89 adopted function prototypes from C++, it replaced the syntax with one requiring the comma. For compatibility, C++98 accepts both C++-stylef(int n...) and C-stylef(int n, ...). The original C++-style grammar is deprecated since C++26.

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

• In other languages

• Español
• 日本語
• Русский
• 中文