va_arg

 

C++

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Concepts library (C++20)

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Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
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Date and time
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initializer_list (C++11)
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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
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strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
common_comparison_category
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pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
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swap
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chars_format (C++17)
Type operations
declval (C++11)
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launder (C++17)

 

Variadic functions

va_start

va_arg

va_copy

va_end

va_list

 

Defined in header <cstdarg>
T va_arg( va_list ap, T );

The va_arg macro expands to an expression of type T that corresponds to the next parameter from the va_list ap.

Prior to calling va_arg, ap must be initialized by a call to either va_start or va_copy, with no intervening call to va_end. Each invocation of the va_arg macro modifies ap to point to the next variable argument.

If the type of the next argument in ap (after promotions) is not compatible with T, the behavior is undefined, unless:

• one type is a signed integer type, the other type is the corresponding unsigned integer type, and the value is representable in both types; or
• one type is pointer to void and the other is a pointer to a character type (char, signed char, or unsigned char).

If va_arg is called when there are no more arguments in ap, the behavior is undefined.

Parameters

ap	-	an instance of the va_list type
T	-	the type of the next parameter in ap

Expanded value

the next variable parameter in ap

Example

Run this code

#include <iostream>
#include <cstdarg>
#include <cmath>
 
double stddev(int count, ...) 
{
    double sum = 0;
    double sum_sq = 0;
    va_list args;
    va_start(args, count);
    for (int i = 0; i < count; ++i) {
        double num = va_arg(args, double);
        sum += num;
        sum_sq += num*num;
    }
    va_end(args);
    return std::sqrt(sum_sq/count - (sum/count)*(sum/count));
}
 
int main() 
{
    std::cout << stddev(4, 25.0, 27.3, 26.9, 25.7) << '\n';
}

Output:

0.920258

See also

va_start	enables access to variadic function arguments (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)