# va_arg

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< cpp‎ | utility‎ | variadic

Defined in header `<cstdarg>` |  |
T va_arg( std::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 <cstdarg>
    #include <cstdio>
    #include <iostream>

    void print_variance(std::size_t count, const char* fmt, ...)
    {
        double sum = 0;
        double sum_sq = 0;
        std::va_list args;
        va_start(args, fmt);
        for (std::size_t i = count; i--;)
        {
            double num = va_arg(args, double);
            sum += num;
            sum_sq += num*num;
        }
        va_end(args);
        std::printf(fmt, sum_sq / count - (sum / count) * (sum / count));
    }

    void nano_printf(const char* fmt, ...)
    {
        std::va_list args;
        va_start(args, fmt);

        for (const char* p = fmt; *p != '\0'; ++p)
        {
            switch (*p)
            {
            case '%':
                switch (*++p) // read format symbol
                {
                    case 'i':
                        std::cout << va_arg(args, int);
                        continue;
                    case 'f':
                        std::cout << va_arg(args, double);
                        continue;
                    case 's':
                        std::cout << va_arg(args, const char*);
                        continue;
                    case 'c':
                        std::cout << static_cast<char>(va_arg(args, int));
                        continue;
                    case '%':
                        std::cout << '%';
                        continue;
                    /* ...more cases... */
                }
                break; // format error...
            case '\n':
                std::cout << '\n';
                continue;
            case '\t':
                std::cout << '\t';
                continue;
            /* ...more cases... */
            }
            std::cout << *p;
        }

        va_end(args);
    }

    int main()
    {
        print_variance(4, "%f\n", 25.0, 27.3, 26.9, 25.7);
        nano_printf("Args: %i%% %c%f %s\n", 42, '#', 3.14, "C++");
    }

Output:

    0.846875
    Args: 42% #3.14 C++

### 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)
C documentation for va_arg

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