va_start

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Utilities library

Language support

Type support (basic types, RTTI)

Library feature-test macros(C++20)

Program utilities

Variadic functions

initializer_list

(C++11)

is_constant_evaluated

(C++20)

is_within_lifetime

(C++26)

source_location

(C++20)

Coroutine support(C++20)

Contract support(C++26)

Three-way comparison

three_way_comparablethree_way_comparable_with

(C++20)(C++20)

strong_ordering

(C++20)

weak_ordering

(C++20)

partial_ordering

(C++20)

common_comparison_category

(C++20)

compare_three_way_result

(C++20)

(C++20)

(C++20)

(C++20)

(C++20)

compare_strong_order_fallback

(C++20)

compare_weak_order_fallback

(C++20)

compare_partial_order_fallback

(C++20)

is_eqis_ltis_lteq

(C++20)(C++20)(C++20)

is_neqis_gtis_gteq

(C++20)(C++20)(C++20)

General utilities

Function objects
Bit manipulation(C++20)
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bitset
hash (C++11)

Relational operators(deprecated in C++20)

rel_ops::operator!=rel_ops::operator>

rel_ops::operator<=rel_ops::operator>=

Integer comparison functions

cmp_equalcmp_lesscmp_less_than

(C++20)(C++20)(C++20)

cmp_not_equalcmp_greatercmp_greater_than

(C++20)(C++20)(C++20)

in_range

(C++20)

Swap andtype operations

swap
ranges::swap (C++20)
exchange (C++14)
declval (C++11)
to_underlying (C++23)

forward (C++11)
forward_like (C++23)
move (C++11)
move_if_noexcept (C++11)
as_const (C++17)

Common vocabulary types

pair
tuple (C++11)
optional (C++17)
any (C++17)
variant (C++17)

tuple_size (C++11)
tuple_element (C++11)
apply (C++17)
make_from_tuple (C++17)
expected (C++23)

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Variadic functions

va_start
va_arg
va_copy (C++11)
va_end
va_list

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Defined in header`<cstdarg>`
void va_start( std::va_list ap, parm_n);

Theva_start macro enables access to the variable arguments following the named argumentparm_n.

va_start should be invoked with an instance to a validva_list objectap before any calls tova_arg.

If theparm_n is apack expansion or an entity resulting from alambda capture, the program is ill-formed, no diagnostic required.

(since C++11)

Ifparm_n is of reference type, or of a type not compatible with the type that results fromdefault argument promotions, the behavior is undefined.

[edit]Parameters

ap	-	an object of theva_list type
parm_n	-	the named parameter preceding the first variable parameter

[edit]Expanded value

(none)

[edit]Notes

va_start is required to supportparm_n with overloadedoperator&.

[edit]Example

Run this code

#include <cstdarg>#include <iostream> int add_nums(int count...){int result=0;    std::va_list args;    va_start(args, count);for(int i=0; i< count;++i)        result+=va_arg(args,int);va_end(args);return result;} int main(){std::cout<< add_nums(4,25,25,50,50)<<'\n';}

Output:

[edit]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 273	C++98	it was unclear whether`va_start` is required to supportparm_ns with overloaded`operator&`	required
LWG 2099	C++98	the behavior was undefined ifparm_n is declared with a function, array, or reference type	the behavior is undefined if parm_n is of reference type

[edit]See also

va_arg	accesses the next variadic function argument (function macro)[edit]
va_end	ends traversal of the variadic function arguments (function macro)[edit]
C documentation forva_start