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# va_arg

From cppreference.com
< c‎ | variadic

Defined in header `<stdarg.h>` |  |
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 `va_arg` is called when there are no more arguments in `ap`, or 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.

### [edit] Parameters

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

### [edit] Expanded value

the next variable parameter in `ap`

### [edit] Example

Run this code

    #include <stdio.h>
    #include <stdarg.h>
    #include <math.h>

    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 sqrt(sum_sq/count - (sum/count)*(sum/count));
    }

    int main(void)
    {
        printf("%f\n", stddev(4, 25.0, 27.3, 26.9, 25.7));
    }

Output:

    0.920258

### [edit] References

- C11 standard (ISO/IEC 9899:2011):

  - 7.16.1.1 The va_arg macro (p: 269-270)

- C99 standard (ISO/IEC 9899:1999):

  - 7.15.1.1 The va_arg macro (p: 249-250)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.8.1.2 The va_arg macro

### [edit] See also

va_copy (C99) | makes a copy of the variadic function arguments (function macro) [edit]
va_end | ends traversal of the variadic function arguments (function macro) [edit]
va_list | holds the information needed by va_start, va_arg, va_end, and va_copy (typedef) [edit]
va_start | enables access to variadic function arguments (function macro) [edit]
C++ documentation for va_arg

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