opencv4.0 获取文件夹下所有图片路径

std::vector<cv::String> filenames; 
cv::String folder = "img";
cv::glob(folder, filenames); 
for (int iFileIndex = 0; iFileIndex < filenames.size(); iFileIndex += 1){
     std::string sInFile = filenames[iFileIndex];
     printf("%s%i%s%i%s\n", "正在执行第", iFileIndex + 1, "个,共", filenames.size(), "个");
     int ipos = sInFile.find_last_of("\\");
     std::string sFile = sInFile.substr(ipos + 1, sInFile.length() - ipos);
}