Variadic arguments

 

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Allows a function to accept any number of extra arguments.

Indicated by a trailing ... (other than one introducing a pack expansion) (since C++11) following the parameter-list of a function declaration.

When the parameter-list is not empty, an optional comma may precede a ... signifying a variadic function. This provides compatibility with C (which added a requirement for a comma when it adopted function prototypes from C++).

// the function declared as follows
int printx(const char* fmt...);
// may be called with one or more arguments:
printx("hello world");
printx("a=%d b=%d", a, b);
 
int printx(const char* fmt, ...); // same as above (extraneous comma is allowed
                                  // for C compatibility)
int printy(..., const char* fmt); // error: ... cannot appear as a parameter
int printz(...); // valid, but the arguments cannot be accessed portably

Note: this is different from a function parameter pack expansion, which is indicated by an ellipsis that is a part of a parameter declarator, rather than an ellipsis that appears after all parameter declarations. Both parameter pack expansion and the "variadic" ellipsis may appear in the declaration of a function template, as in the case of std::is_function.

(since C++11)

Default conversions

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

std::nullptr_t is converted to void*

(since C++11)

• float arguments are converted to double as in floating-point promotion
• bool, char, short, and unscoped enumerations are converted to int or wider integer types as in integer promotion

Only arithmetic, enumeration, pointer, pointer to member, and class type arguments (after conversion) are allowed. However, non-POD class types (until C++11)class types with an eligible non-trivial copy constructor, an eligible non-trivial move constructor, or a non-trivial destructor, together with scoped enumerations (since C++11), are conditionally-supported in potentially-evaluated calls with implementation-defined semantics (these types are always supported in unevaluated calls).

Because variadic parameters have the lowest rank for the purpose of overload resolution, they are commonly used as the catch-all fallbacks in SFINAE.

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

Defined in header <cstdarg>
va_start	enables access to variadic function arguments (function macro)
va_arg	accesses the next variadic function argument (function macro)
va_copy (C++11)	makes a copy of the variadic function arguments (function macro)
va_end	ends traversal of the variadic function arguments (function macro)
va_list	holds the information needed by va_start, va_arg, va_end, and va_copy (typedef)

The behavior of the va_start macro is undefined if the last parameter before the ellipsis has reference type, or has type that is not compatible with the type that results from default argument promotions.

If the a pack expansion or an entity resulting from a lambda capture is used as the last parameter in va_start, the program is ill-formed, no diagnostic required.

(since C++11)

Alternatives

Variadic templates can also be used to create functions that take variable number of arguments. They are often the better choice because they do not impose restrictions on the types of the arguments, do not perform integral and floating-point promotions, and are type safe. If all variable arguments share a common type, a std::initializer_list provides a convenient mechanism (albeit with a different syntax) for accessing variable arguments. In this case however the arguments cannot be modified since std::initializer_list can only provide a const pointer to its elements.

(since C++11)

Notes

In the C programming language until C23, at least one named parameter must appear before the ellipsis parameter, so R 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 in SFINAE, exploiting the lowest priority of the ellipsis conversion in overload 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++-style f(int n...) and C-style f(int n, ...).

The comma can be used in abbreviated function templates to make the ellipsis signify a variadic function instead of a variadic template: void f1(auto...); // same as template<class... Ts> void f3(Ts...) void f2(auto, ...); // same as template<class T> void f3(T...)

(since C++20)

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
CWG 506	C++98	passing non-POD class arguments to an ellipsis resulted in undefined behavior	passing such arguments is conditionally-supported with implementation-defined semantics
CWG 634	C++98	conditionally-supported class types made some SFINAE idioms not work	always supported if unevaluated
CWG 2247	C++11	no restriction on passing parameter pack or lambda capture to va_start	made ill-formed, no diagnostic required
CWG 2347	C++11	it was unclear whether scoped enumerations passed to an ellipsis are subject to default argument promotions	passing scoped enumerations is conditionally-supported with implementation-defined semantics