Fastest way to check if a file exist using standard C++/C++11,14,17/C?

Question

I would like to find the fastest way to check if a file exists in standard C++11, 14, 17, or C. I have thousands of files and before doing something on them I need to check if all of them exist. What can I write instead of /* SOMETHING */ in the following function?

inline bool exist(const std::string& name) {     /* SOMETHING */ } 

Answer 1

Well I threw together a test program that ran each of these methods 100,000 times, half on files that existed and half on files that didn't.

#include <sys/stat.h> #include <unistd.h> #include <string> #include <fstream>  inline bool exists_test0 (const std::string& name) {     ifstream f(name.c_str());     return f.good(); }  inline bool exists_test1 (const std::string& name) {     if (FILE *file = fopen(name.c_str(), "r")) {         fclose(file);         return true;     } else {         return false;     }    }  inline bool exists_test2 (const std::string& name) {     return ( access( name.c_str(), F_OK ) != -1 ); }  inline bool exists_test3 (const std::string& name) {   struct stat buffer;      return (stat (name.c_str(), &buffer) == 0);  } 

Results for total time to run the 100,000 calls averaged over 5 runs,

Method	Time
exists_test0 (ifstream)	0.485s
exists_test1 (FILE fopen)	0.302s
exists_test2 (posix access())	0.202s
exists_test3 (posix stat())	0.134s

The stat() function provided the best performance on my system (Linux, compiled with g++), with a standard fopen call being your best bet if you for some reason refuse to use POSIX functions.

Answer 2

Remark : in C++14 and as soon as the filesystem TS will be finished and adopted, the solution will be to use:

std::experimental::filesystem::exists("helloworld.txt"); 

and since C++17, only:

std::filesystem::exists("helloworld.txt");