How to get list of files with a specific extension in a given folder?

a C++17 code

#include <fstream>
#include <iostream>
#include <filesystem>
namespace fs = std::filesystem;

int main()
{
    std::string path("/your/dir/");
    std::string ext(".sample");
    for (auto &p : fs::recursive_directory_iterator(path))
    {
        if (p.path().extension() == ext)
            std::cout << p.path().stem().string() << '\n';
    }
    return 0;
}

#define BOOST_FILESYSTEM_VERSION 3
#define BOOST_FILESYSTEM_NO_DEPRECATED 
#include <boost/filesystem.hpp>

namespace fs = boost::filesystem;

/**
 * \brief   Return the filenames of all files that have the specified extension
 *          in the specified directory and all subdirectories.
 */
std::vector<fs::path> get_all(fs::path const & root, std::string const & ext)
{
    std::vector<fs::path> paths;

    if (fs::exists(root) && fs::is_directory(root))
    {
        for (auto const & entry : fs::recursive_directory_iterator(root))
        {
            if (fs::is_regular_file(entry) && entry.path().extension() == ext)
                paths.emplace_back(entry.path().filename());
        }
    }

    return paths;
}             

Get list of files and process each file and loop through them and store back in different folder

void getFilesList(string filePath,string extension, vector<string> & returnFileName)
{
    WIN32_FIND_DATA fileInfo;
    HANDLE hFind;   
    string  fullPath = filePath + extension;
    hFind = FindFirstFile(fullPath.c_str(), &fileInfo);
    if (hFind != INVALID_HANDLE_VALUE){
        returnFileName.push_back(filePath+fileInfo.cFileName);
        while (FindNextFile(hFind, &fileInfo) != 0){
            returnFileName.push_back(filePath+fileInfo.cFileName);
        }
    }
}

USE: you can use like this load all the files from folder and loop through one by one

String optfileName ="";        
String inputFolderPath =""; 
String extension = "*.jpg*";
getFilesList(inputFolderPath,extension,filesPaths);
vector<string>::const_iterator it = filesPaths.begin();
while( it != filesPaths.end())
{
    frame = imread(*it);//read file names
        //doyourwork here ( frame );
    sprintf(buf, "%s/Out/%d.jpg", optfileName.c_str(),it->c_str());
    imwrite(buf,frame);   
    it++;
}

On windows you do something like this:

void listFiles( const char* path )
{
   struct _finddata_t dirFile;
   long hFile;

   if (( hFile = _findfirst( path, &dirFile )) != -1 )
   {
      do
      {
         if ( !strcmp( dirFile.name, "."   )) continue;
         if ( !strcmp( dirFile.name, ".."  )) continue;
         if ( gIgnoreHidden )
         {
            if ( dirFile.attrib & _A_HIDDEN ) continue;
            if ( dirFile.name[0] == '.' ) continue;
         }

         // dirFile.name is the name of the file. Do whatever string comparison 
         // you want here. Something like:
         if ( strstr( dirFile.name, ".txt" ))
            printf( "found a .txt file: %s", dirFile.name );

      } while ( _findnext( hFile, &dirFile ) == 0 );
      _findclose( hFile );
   }
}

On Posix, like Linux or OsX:

void listFiles( const char* path )
{
   DIR* dirFile = opendir( path );
   if ( dirFile ) 
   {
      struct dirent* hFile;
      errno = 0;
      while (( hFile = readdir( dirFile )) != NULL ) 
      {
         if ( !strcmp( hFile->d_name, "."  )) continue;
         if ( !strcmp( hFile->d_name, ".." )) continue;

         // in linux hidden files all start with '.'
         if ( gIgnoreHidden && ( hFile->d_name[0] == '.' )) continue;

         // dirFile.name is the name of the file. Do whatever string comparison 
         // you want here. Something like:
         if ( strstr( hFile->d_name, ".txt" ))
            printf( "found an .txt file: %s", hFile->d_name );
      } 
      closedir( dirFile );
   }
}