std::filesystem::copy_file(3)

NAME
std::filesystem::copy_file - std::filesystem::copy_file

Synopsis
   Defined in header <filesystem>
   bool copy_file( const std::filesystem::path& from, (1) (since C++17)
                   const std::filesystem::path& to );
   bool copy_file( const std::filesystem::path& from,
                   const std::filesystem::path& to, (2) (since C++17)
                   std::error_code& ec );
   bool copy_file( const std::filesystem::path& from,
                   const std::filesystem::path& to, (3) (since C++17)
                   std::filesystem::copy_options options );
   bool copy_file( const std::filesystem::path& from,
                   const std::filesystem::path& to, (4) (since C++17)
                   std::filesystem::copy_options options,
                   std::error_code& ec );
   1,2) The default, equivalent to (3,4) with copy_options::none used as options.
   3,4) Copies a single file from from to to, using the copy options indicated by
   options. The behavior is undefined if there is more than one option in any of the
   copy_options option group present in options (even in the groups not relevant to
   filesystem::copy_file).
     * If !filesystem::is_regular_file(from) (either because the source file doesn't
       exist or because it is not a regular file), report an error.
     * Otherwise, if the destination file does not exist,
     * copies the contents and the attributes of the file to which from resolves to the
       file to which to resolves (symlinks are followed).
     * Otherwise, if the destination file already exists,
     * report an error if any of the following is true:
     * to and from are the same as determined by filesystem::equivalent(from, to);
     * to is not a regular file as determined by !filesystem::is_regular_file(to);
     * none of the filesystem::copy_file control options are set in options.
     * Otherwise, if copy_options::skip_existing is set in options, do nothing.
     * Otherwise, if copy_options::overwrite_existing is set in options, copy the
       contents and the attributes of the file to which from resolves to the file to
       which to resolves.
     * Otherwise, if copy_options::update_existing is set in options, only copy the
       file if from is newer than to, as defined by filesystem::last_write_time().
   The non-throwing overloads return false if an error occurs.

Parameters
   from - path to the source file
   to - path to the target file
   ec - out-parameter for error reporting in the non-throwing overload

Return value
   true if the file was copied, false otherwise.

Exceptions
   Any overload not marked noexcept may throw std::bad_alloc if memory allocation
   fails.
   1,3) Throws std::filesystem::filesystem_error on underlying OS API errors,
   constructed with from as the first path argument, to as the second path argument,
   and the OS error code as the error code argument.
   2,4) 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
   The functions involve at most one direct or indirect call to filesystem::status(to)
   (used both to determine if the file exists, and, for
   filesystem::copy_options::update_existing option, its last write time).
   Error is reported when filesystem::copy_file is used to copy a directory: use
   filesystem::copy for that.
   filesystem::copy_file follows symlinks: use filesystem::copy_symlink or
   filesystem::copy with filesystem::copy_options::copy_symlinks for that.

Example
// Run this code
 #include <filesystem>
 #include <fstream>
 #include <iostream>
 namespace fs = std::filesystem;
 int main()
 {
     fs::create_directory("sandbox");
     std::ofstream("sandbox/file1.txt").put('a');
     fs::copy_file("sandbox/file1.txt", "sandbox/file2.txt");
     // now there are two files in sandbox:
     std::cout << "file1.txt holds: "
               << std::ifstream("sandbox/file1.txt").rdbuf() << '\n';
     std::cout << "file2.txt holds: "
               << std::ifstream("sandbox/file2.txt").rdbuf() << '\n';
     // fail to copy directory
     fs::create_directory("sandbox/abc");
     try
     {
         fs::copy_file("sandbox/abc", "sandbox/def");
     }
     catch (fs::filesystem_error& e)
     {
         std::cout << "Could not copy sandbox/abc: " << e.what() << '\n';
     }
     fs::remove_all("sandbox");
 }

Possible output:
 file1.txt holds: a
 file2.txt holds: a
 Could not copy sandbox/abc: copy_file: Is a directory: "sandbox/abc", "sandbox/def"
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3014 C++17 error_code overload marked noexcept but can noexcept removed
                       allocate memory

See also
   copy_options specifies semantics of copy operations
   (C++17) (enum)
   copy_symlink copies a symbolic link
   (C++17) (function)
   copy copies files or directories
   (C++17) (function)