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:
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.
ap | - | an instance of the va_list type |
T | - | the type of the next parameter in ap |
the next variable parameter in ap
.
#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
enables access to variadic function arguments (function macro) |
|
(C++11) | makes a copy of the variadic function arguments (function macro) |
ends traversal of the variadic function arguments (function macro) |
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