Variadic arguments

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++20)
noexcept specifier (C++11)
Exceptions
throw-expression	try-catch block
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
decltype (C++11) auto (C++11) alignas (C++11)	const/volatile constexpr (C++11)
Storage duration specifiers
Initialization
Default initialization Value initialization Zero initialization Copy initialization Direct initialization	Aggregate initialization     List initialization (C++11) Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Functions

Declarations

Function declaration

Function parameter list

Function definition

Default arguments

Variadic arguments

inline specifier

Lambda expressions (C++11)

Coroutines (C++20)

Function calls

Argument-Dependent Lookup (ADL)

Function-call operator

Function objects

Overloading

Overload resolution

Operator overloading

Address of an overload set

 

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 (since C++11) 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, ...).

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

See also

C documentation for Variadic arguments