Fastest way to check if a file exist using standard C++/C++11/C?
I would like to find the fastest way to check if a file exist in standard C++11, C++, 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 */
}
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>
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 exists_test0 (ifstream): **0.485s**
Method exists_test1 (FILE fopen): **0.302s**
Method exists_test2 (posix access()): **0.202s**
Method 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.
I use this piece of code, it works OK with me so far. This does not use many fancy features of C++:
bool is_file_exist(const char *fileName)
{
std::ifstream infile(fileName);
return infile.good();
}
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 hopefully in C++17, only:
std::filesystem::exists("helloworld.txt");
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