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std::basic_fstream::open(3)    C++ Standard Libary   std::basic_fstream::open(3)

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

Synopsis
	void open( const char* filename,

		   std::ios_base::openmode mode 		      (1)

		       = std::ios_base::in | std::ios_base::out );
	void open( const std::filesystem::path::value_type* filename,

		   std::ios_base::openmode mode 		      (2) (since
     C++17)

		       = std::ios_base::in | std::ios_base::out );
	void open( const std::string& filename,

		   std::ios_base::openmode mode 		      (3) (since
     C++11)

		       = std::ios_base::in | std::ios_base::out );
	void open( const std::filesystem::path& filename,

		   std::ios_base::openmode mode 		      (4) (since
     C++17)

		       = std::ios_base::in | std::ios_base::out );

	Opens and associates the file with name filename with the file stream.

	Calls clear() on success. Calls setstate(failbit) on failure.

	1,2)  Effectively  calls  rdbuf()->open(filename,  mode)  (see	std::ba-
     sic_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 a BitmaskType, the follow-
     ing constants
		   are defined:

		   Constant	     Explanation
		   app		     seek to the end of stream before each write
	mode	 - 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
		   noreplace (C++23) open in exclusive mode

Return value
	(none)

Example
     // Run this code

      #include <fstream>
      #include <iostream>
      #include <string>

      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';
      }

	Defect reports

	The  following	behavior-changing  defect  reports were applied retroac-
     tively to
	previously published C++ standards.

	  DR	Applied to	       Behavior as published		    Cor-
     rect behavior
	LWG  22   C++98       it was unclear how error state changes upon a  the
     error state is
			   successful  open				     un-
     changed
	LWG  409 C++98	    the error state was unchanged upon a	   it is
     cleared^[1]
			   successful open
	LWG 460 C++98	   the default argument of mode in overload (1)   added
			   was missing (it is present in the synopsis)

	 1. a The resolution of LWG issue #22 is overriden.

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  se-
     quence
		(public member function of std::basic_filebuf<CharT,Traits>)

http://cppreference.com 	   2024.06.10	     std::basic_fstream::open(3)
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