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std::basic_ostream::sentry(3) C++ Standard Libarystd::basic_ostream::sentry(3)

NAME
       std::basic_ostream::sentry - std::basic_ostream::sentry

Synopsis
	  class	sentry;

	  An  object  of  class	 basic_ostream::sentry is constructed in local
       scope at	the
	  beginning of each member function of	std::basic_ostream  that  per-
       forms output (both
	  formatted  and  unformatted).	 Its  constructor  prepares the	output
       stream: checks if
	  the stream is	already	in a failed state, flushes the tie()'d	output
       streams,	and
	  performs  other implementation-defined tasks if necessary. Implemen-
       tation-defined
	  cleanup, as well as flushing of the output stream if	necessary,  is
       performed in the
	  destructor,  so  that	 it  is	guaranteed to happen if	exceptions are
       thrown during
	  output.

Member functions
	  constructor	constructs the	sentry	object.	 All  the  preparation
       tasks are done here
			(public	member function)
			finalizes  the stream object after formatted output or
       after exception,
	  destructor	if necessary
			(public	member function)
	  operator=	the assignment operator	is deleted
			(public	member function)
	  operator bool	checks if the preparation of  the  stream  object  was
       successful
			(public	member function)

       std::basic_ostream::sentry::sentry

	  explicit sentry( std::basic_ostream<CharT,Traits>& os	);

	  Prepares the stream for formatted output.

	  If os.good() is false, returns. Otherwise, if	os.tie() is not	a null
       pointer,	calls
	  os.tie()->flush()  to	 synchronize the output	sequence with external
       streams.	During
	  preparation, the constructor may call	setstate(failbit)  (which  may
       throw
	  std::ios_base::failure).

	  If  after preparation	is completed, os.good()	== true, then any sub-
       sequent calls to
	  operator bool	will return true.

Parameters
	  os - output stream to	prepare

Exceptions
	  std::ios_base::failure if the	end of file condition occurs.

       std::basic_ostream::sentry::~sentry

	  ~sentry();

	  If (os.flags() &  std::ios_base::unitbuf)  &&	 !std::uncaught_excep-
       tion() && os.good())
	  is  true,  calls os.rdbuf()->pubsync(). If that function returns -1,
       sets badbit in
	  os.rdstate() without propagating an exception.

       std::basic_ostream::sentry::operator bool

	  explicit operator bool() const;

	  Checks whether the preparation of the	output stream was successful.

Parameters
	  (none)

Return value
	  true if the preparation of the output	stream was  successful,	 false
       otherwise.

Example
       // Run this code

	#include <iostream>
	#include <sstream>

	struct Foo
	{
	    char n[6];
	};

	std::ostream& operator<<(std::ostream& os, Foo&	f)
	{
	    std::ostream::sentry s(os);
	    if (s) {
		os.write(f.n, 5);
	    }
	    return os;
	}

	int main()
	{
	    Foo	f = { "abcde" };
	    std::cout << f << '\n';
	}

Output:
	abcde

See also
	  operator<< inserts formatted data
		     (public member function)

http://cppreference.com		  2022.07.31	 std::basic_ostream::sentry(3)

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