std::basic_fstream::open(3)

NAME
std::basic_fstream::open - std::basic_fstream::open

Synopsis
   void open( const char *filename, (1)
   ios_base::openmode mode = ios_base::in|ios_base::out );
   void open( const std::filesystem::path::value_type *filename, (2) (since C++17)
   ios_base::openmode mode = ios_base::in|ios_base::out );
   void open( const std::string &filename, (3) (since C++11)
   ios_base::openmode mode = ios_base::in|ios_base::out );
   void open( const std::filesystem::path &filename, (4) (since C++17)
   ios_base::openmode mode = ios_base::in|ios_base::out );
   Opens and associates the file with name filename with the file stream.
   Calls setstate(failbit) on failure.
   Calls clear() on success. (since C++11)
   1-2) Effectively calls rdbuf()->open(filename, mode). (see std::basic_filebuf::open
   for the details on the effects of that call).
   Overload (2) is only provided if std::filesystem::path::value_type is not char.
   (since C++17)
   3-4) Effectively calls (1-2) as if by open(filename.c_str(), mode).

Parameters
   filename - the name of the file to be opened
              specifies stream open mode. It is bitmask type, the following constants
              are defined:
              Constant Explanation
   mode - app seek to the end of stream before each write
              binary open in binary mode
              in open for reading
              out open for writing
              trunc discard the contents of the stream when opening
              ate seek to the end of stream immediately after open

Return value
   (none)

Example
// Run this code
 #include <string>
 #include <fstream>
 #include <iostream>
 int main()
 {
     std::string filename = "example.123";
     std::fstream fs;
     fs.open(filename);
     if(!fs.is_open())
     {
        fs.clear();
        fs.open(filename, std::ios::out); //Create file.
        fs.close();
        fs.open(filename);
     }
     std::cout << std::boolalpha;
     std::cout << "fs.is_open() = " << fs.is_open() << '\n';
     std::cout << "fs.good() = " << fs.good() << '\n';
 }

See also
   is_open checks if the stream has an associated file
           (public member function)
   close closes the associated file
           (public member function)
   open opens a file and configures it as the associated character sequence
           (public member function of std::basic_filebuf<CharT,Traits>)