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From elsewhere, but even more complete:

Yes, Boost is great, but it's a bit bloaty. So, just for completenessapplied to reading images in a directory for OpenCV:

// you need these includes for the function
//#include <windows.h> // for windows systems
#include <dirent.h> // for linux systems
#include <sys/stat.h> // for linux systems
#include <algorithm>    // std::sort
#include <opencv2/opencv.hpp>
#include <iostream>   //cout

using namespace std;

/* Returns a list of files in a directory (except the ones that begin with a dot) */
int readFilenames(std::vector<string> &filenames, const string &directory)
{
#ifdef WINDOWS
    HANDLE dir;
    WIN32_FIND_DATA file_data;

    if ((dir = FindFirstFile((directory + "/*").c_str(), &file_data)) == INVALID_HANDLE_VALUE)
        return; /* No files found */

    do {
        const string file_name = file_data.cFileName;
        const string full_file_name = directory + "/" + file_name;
        const bool is_directory = (file_data.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY) != 0;

        if (file_name[0] == '.')
            continue;

        if (is_directory)
            continue;

        filenames.push_back(full_file_name);
    } while (FindNextFile(dir, &file_data));

    FindClose(dir);
#else
    DIR *dir;
    class dirent *ent;
    class stat st;

    dir = opendir(directory.c_str());
    while ((ent = readdir(dir)) != NULL) {
        const string file_name = ent->d_name;
        const string full_file_name = directory + "/" + file_name;

        if (file_name[0] == '.')
            continue;

        if (stat(full_file_name.c_str(), &st) == -1)
            continue;

        const bool is_directory = (st.st_mode & S_IFDIR) != 0;

        if (is_directory)
            continue;

//        filenames.push_back(full_file_name); // returns full path
        filenames.push_back(file_name); // returns just filename
    }
    closedir(dir);
#endif
    std::sort (filenames.begin(), filenames.end()); //optional, sort the filenames
    return(filenames.size()); //Return how many we found
} // GetFilesInDirectory


void help(const char **argv) {
  cout << "\n\n"
       << "Call:\n" << argv[0] << " <directory path>\n\n"
       << "Given a directory of images, create a vector of\n"
       << "their names, read and display them. Filter out\n"
       << "non-images\n"
       << endl;
}

int main( int argc, const char** argv )
{
    if(argc != 2) {
        cerr << "\nIncorrect number of parameters: " << argc << ", should be 2\n" << endl;
        help(argv);
        return -1;
    }
    string folder = argv[1];
    cout << "Reading in directory " << folder << endl;
    vector<string> filenames;

    int num_files = readFilenames(filenames, folder);
    cout << "Number of files = " << num_files << endl;
    cv::namedWindow( "image", 1 );
    for(size_t i = 0; i < filenames.size(); ++i)
    {
        cout << folder + filenames[i] << " #" << i << endl;
        cv::Mat src = cv::imread(folder + filenames[i]);

        if(!src.data) { //Protect against no file
            cerr << folder + filenames[i] << ", file #" << i << ", is not an image" << endl;
            continue;
        }
        cv::imshow("image", src);
        cv::waitKey(250); //For fun, wait 250ms, or a quarter of a second, but you can put in "0" for no wait or -1 to wait for keypresses
        /* do whatever you want with your images here */
    }
}