std::basic_filebuf::open(3)

NAME
std::basic_filebuf::open - std::basic_filebuf::open

Synopsis
   basic_filebuf* open( const char* s, std::ios_base::openmode mode (1)
   );
   basic_filebuf* open( const std::string& str, (2) (since C++11)
   std::ios_base::openmode mode );
   basic_filebuf* open( const std::filesystem::path& p, (3) (since C++17)
                        std::ios_base::openmode mode );
   basic_filebuf* open( const std::filesystem::path::value_type* s, (4) (since C++17)
                        std::ios_base::openmode mode );
   If the associated file was already open (is_open() != false), returns a null pointer
   right away.
   Otherwise, opens the file with the given name (s
   , p.c_str()
   (since C++17) or str.c_str(), depending on the overload). std::ios_base::openmode
   values may be written as, e.g., std::ios_base::out | std::ios_base::app.
   Overload (4) is only provided if std::filesystem::path::value_type is (since C++17)
   not char.
   The file is opened as if by calling std::fopen with the second argument (file access
   mode) determined by the result of mode & ~std::ios_base::ate as follows, open()
   fails if the result is not some combination of flags shown in the table:
        mode & ~std::ios_base::ate  std::fopen  Action if file Action if file
   binary in out trunc app noreplace access already exists does not exist
                        (since C++23) mode
   - + - - - - "r" Failure to
   + + - - - - "rb" Read from start open
   - + + - - - "r+" Error
   + + + - - - "r+b"
   - - + - - - "w"
   - - + + - -
   + - + - - - "wb" Destroy contents Create new
   + - + + - -
   - + + + - - "w+"
   + + + + - - "w+b"
   - - + - - + "wx"
   - - + + - +
   + - + - - + "wbx" Failure to open Create new
   + - + + - +
   - + + + - + "w+x"
   + + + + - + "w+bx"
   - - + - + - "a"
   - - - - + -
   + - + - + - "ab"
   + - - - + - Write to end Create new
   - + + - + - "a+"
   - + - - + -
   + + + - + - "a+b"
   + + - - + -
   If the open operation succeeds and (openmode & std::ios_base::ate) != 0 (the ate bit
   is set), repositions the file position to the end of file, as if by calling
   std::fseek(file, 0, SEEK_END), where file is the pointer returned by calling
   std::fopen. If the repositioning fails, calls close() and returns a null pointer to
   indicate failure.

Parameters
   s, str, p - the file name to open; s must point to a null-terminated string
   openmode - the file opening mode, a binary OR of the std::ios_base::openmode modes

Return value
   this on success, a null pointer on failure.

Notes
   open() is typically called through the constructor or the open() member function of
   std::basic_fstream.

Example
// Run this code
 #include <fstream>
 #include <iostream>
 int main()
 {
     std::string filename = "Test.b";
     std::filebuf fb;
     // prepare a file to read
     double d = 3.14;
     if (!fb.open(filename, std::ios::binary | std::ios::out))
     {
         std::cout << "Open file " << filename << " for write failed\n";
         return 1;
     }
     fb.sputn(reinterpret_cast<char*>(&d), sizeof d);
     fb.close();
     // open file for reading
     double d2 = 0.0;
     if (!fb.open(filename, std::ios::binary | std::ios::in))
     {
         std::cout << "Open file " << filename << " for read failed\n";
         return 1;
     }
     auto got = fb.sgetn(reinterpret_cast<char*>(&d2), sizeof d2);
     if (sizeof(d2) != got)
         std::cout << "Read of " << filename << " failed\n";
     else
         std::cout << "Read back from file: " << d2 << '\n';
 }

Output:
 Read back from file: 3.14
   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 596 C++98 open() could not open files in append can open in append mode
                      mode

See also
   is_open checks if the associated file is open
           (public member function)
   close flushes the put area buffer and closes the associated file
           (public member function)