std::filesystem::space_info(3)

NAME
std::filesystem::space_info - std::filesystem::space_info

Synopsis
   Defined in header <filesystem>
   struct space_info {
       std::uintmax_t capacity;
       std::uintmax_t free; (since C++17)
       std::uintmax_t available;
   };
   Represents the filesystem information as determined by filesystem::space.

Member objects
   capacity total size of the filesystem, in bytes
             (public member object)
   free free space on the filesystem, in bytes
             (public member object)
             free space available to a non-privileged process (may be equal or less
   available than free)
             (public member object)

Non-member functions
   operator== compares two space_infos
   (C++20) (function) operator==(std::filesystem::space_info)
   friend bool operator==( const space_info&, const space_info& ) = (since C++20)
   default;
   Checks if capacity, free and available of both arguments are equal respectively.
   This function is not visible to ordinary unqualified or qualified lookup, and can
   only be found by argument-dependent lookup when std::filesystem::space_info is an
   associated class of the arguments.
   The != operator is synthesized from operator==.

Example
// Run this code
 #include <cstdint>
 #include <filesystem>
 #include <iostream>
 std::uintmax_t disk_usage_percent(const std::filesystem::space_info& si,
                        bool is_privileged = false) noexcept
 {
     if (constexpr std::uintmax_t X(-1);
         si.capacity == 0 || si.free == 0 || si.available == 0 ||
         si.capacity == X || si.free == X || si.available == X
     )
         return 100;
     std::uintmax_t unused_space = si.free, capacity = si.capacity;
     if (!is_privileged)
     {
         const std::uintmax_t privileged_only_space = si.free - si.available;
         unused_space -= privileged_only_space;
         capacity -= privileged_only_space;
     }
     const std::uintmax_t used_space{capacity - unused_space};
     return 100 * used_space / capacity;
 }
 void print_disk_space_info(auto const& dirs, int width = 14)
 {
     (std::cout << std::left).imbue(std::locale("en_US.UTF-8"));
     for (const auto s : {"Capacity", "Free", "Available", "Use%", "Dir"})
         std::cout << "│ " << std::setw(width) << s << ' ';
     for (std::cout << '\n'; auto const& dir : dirs)
     {
         std::error_code ec;
         const std::filesystem::space_info si = std::filesystem::space(dir, ec);
         for (auto x : {si.capacity, si.free, si.available, disk_usage_percent(si)})
             std::cout << "│ " << std::setw(width) << static_cast<std::intmax_t>(x) << ' ';
         std::cout << "│ " << dir << '\n';
     }
 }
 int main()
 {
     const auto dirs = {"/dev/null", "/tmp", "/home", "/proc", "/null"};
     print_disk_space_info(dirs);
 }

Possible output:
 │ Capacity │ Free │ Available │ Use% │ Dir
 │ 84,417,331,200 │ 42,732,986,368 │ 40,156,028,928 │ 50 │ /dev/null
 │ 84,417,331,200 │ 42,732,986,368 │ 40,156,028,928 │ 50 │ /tmp
 │ -1 │ -1 │ -1 │ 100 │ /home
 │ 0 │ 0 │ 0 │ 100 │ /proc
 │ -1 │ -1 │ -1 │ 100 │ /null

See also
   space determines available free space on the file system
   (C++17) (function)