std::filesystem::create_hard_link(3)

NAME
std::filesystem::create_hard_link - std::filesystem::create_hard_link

Synopsis
   Defined in header <filesystem>
   void create_hard_link( const std::filesystem::path& target, (1) (since C++17)
                        const std::filesystem::path& link );
   void create_hard_link( const std::filesystem::path& target,
                        const std::filesystem::path& link, (2) (since C++17)
                        std::error_code& ec ) noexcept;
   Creates a hard link link with its target set to target as if by POSIX link(): the
   pathname target must exist.
   Once created, link and target are two logical names that refer to the same file
   (they are equivalent). Even if the original name target is deleted, the file
   continues to exist and is accessible as link.

Parameters
   target - path of the file or directory to link to
   link - path of the new hard link
   ec - out-parameter for error reporting in the non-throwing overload

Return value
   (none)

Exceptions
   Any overload not marked noexcept may throw std::bad_alloc if memory allocation
   fails.
   1) Throws std::filesystem::filesystem_error on underlying OS API errors, constructed
   with target as the first path argument, link as the second path argument, and the OS
   error code as the error code argument.
   2) Sets a std::error_code& parameter to the OS API error code if an OS API call
   fails, and executes ec.clear() if no errors occur.

Notes
   Some operating systems do not support hard links at all or support them only for
   regular files.
   Some file systems do not support hard links regardless of the operating system: the
   FAT file system used on memory cards and flash drives, for example.
   Some file systems limit the number of links per file.
   Hardlinking to directories is typically restricted to the superuser.
   Hard links typically cannot cross filesystem boundaries.
   The special pathname dot (".") is a hard link to its parent directory. The special
   pathname dot-dot ".." is a hard link to the directory that is the parent of its
   parent.

Example
// Run this code
 #include <filesystem>
 #include <fstream>
 #include <iostream>
 namespace fs = std::filesystem;
 int main()
 {
     fs::create_directories("sandbox/subdir");
     std::ofstream("sandbox/a").put('a'); // create regular file
     fs::create_hard_link("sandbox/a", "sandbox/b");
     fs::remove("sandbox/a");
     // read from the original file via surviving hard link
     char c = std::ifstream("sandbox/b").get();
     std::cout << c << '\n';
     fs::remove_all("sandbox");
 }

Output:
 a

See also
   create_symlink
   create_directory_symlink creates a symbolic link
   (C++17) (function)
   (C++17)
   hard_link_count returns the number of hard links referring to the specific
   (C++17) file
                        (function)