Variadic functions

 

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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) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)     type_order (C++26) 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) 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 and type 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)

 

Variadic functions

va_start

va_arg

va_copy (C++11)

va_end

va_list

 

Variadic functions are functions (e.g. std::printf) which take a variable number of arguments.

To declare a variadic function, an ellipsis appears as the last parameter, e.g. int printf(const char* format, ...);. The comma preceding the ellipsis can be omitted(deprecated in C++26). See variadic arguments for additional detail on the syntax, automatic argument conversions and the alternatives.

To access the variadic arguments from the function body, the following library facilities are provided:

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)

Example

Run this code

#include <cstdarg>
#include <iostream>

void simple_printf(const char* fmt...) // C-style "const char* fmt, ..." is also valid
{
    va_list args;
    va_start(args, fmt);

    while (*fmt != '\0')
    {
        if (*fmt == 'd')
        {
            int i = va_arg(args, int);
            std::cout << i << '\n';
        }
        else if (*fmt == 'c')
        {
            // note automatic conversion to integral type
            int c = va_arg(args, int);
            std::cout << static_cast<char>(c) << '\n';
        }
        else if (*fmt == 'f')
        {
            double d = va_arg(args, double);
            std::cout << d << '\n';
        }
        ++fmt;
    }

    va_end(args);
}

int main()
{
    simple_printf("dcff", 3, 'a', 1.999, 42.5); 
}

Output:

3
a
1.999
42.5

See also

C documentation for Variadic functions