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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 fol-
       lowing 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
       Correct behavior
	  LWG 22  C++98	     it	was unclear how	error  state  changes  upon  a
       the error state is
			     successful					  open
       unchanged
	  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.  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
       sequence
		  (public member function of std::basic_filebuf<CharT,Traits>)

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

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