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Defined in header <cstdarg> | ||
T va_arg( std::va_list ap, T); | ||
Theva_arg macro expands to an expression of typeT that corresponds to the next parameter from theva_listap.
Prior to callingva_arg,ap must be initialized by a call to eitherva_start orva_copy, with no intervening call tova_end. Each invocation of theva_arg macro modifiesap to point to the next variable argument.
If the type of the next argument inap (after promotions) is notcompatible withT, the behavior is undefined, unless:
Ifva_arg is called when there are no more arguments inap, the behavior is undefined.
Contents |
| ap | - | an instance of theva_list type |
| T | - | the type of the next parameter inap |
The next variable parameter inap.
#include <cstdarg>#include <cstdio>#include <iostream> void print_variance(std::size_t count,constchar* 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(constchar* fmt, ...){ std::va_list args;va_start(args, fmt); for(constchar* 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,constchar*);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:
0.846875Args: 42% #3.14 C++
| enables access to variadic function arguments (function macro)[edit] | |
(C++11) | makes a copy of the variadic function arguments (function macro)[edit] |
| ends traversal of the variadic function arguments (function macro)[edit] | |
C documentation forva_arg | |
