Listing only folders in directory

You could use opendir() and readdir() to list directories and subdirectories. The following example prints all subdirectories inside the current path:

#include <dirent.h>
#include <stdio.h>

int main()
{
    const char* PATH = ".";

    DIR *dir = opendir(PATH);

    struct dirent *entry = readdir(dir);

    while (entry != NULL)
    {
        if (entry->d_type == DT_DIR)
            printf("%s\n", entry->d_name);

        entry = readdir(dir);
    }

    closedir(dir);

    return 0;
}

Using the C++17 std::filesystem library:

std::vector<std::string> get_directories(const std::string& s)
{
    std::vector<std::string> r;
    for(auto& p : std::filesystem::recursive_directory_iterator(s))
        if (p.is_directory())
            r.push_back(p.path().string());
    return r;
}