va_arg
From cppreference.com
Defined in header <cstdarg>
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T va_arg( std::va_list ap, T ); |
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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
If the type of the next argument in ap (after promotions) is not compatible with T
- 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
| enables access to variadic function arguments (function macro) |
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(C++11)
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makes a copy of the variadic function arguments (function macro) |
| ends traversal of the variadic function arguments (function macro) |
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C documentation for va_arg
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