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


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


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 performs 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. Implementation-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

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)


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

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


os - output stream to prepare


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



If (os.flags() & std::ios_base::unitbuf) && !std::uncaught_exception() && 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.



Return value

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


// 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';



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 397 C++98 the destructor might call os.flush(), which the exception is not
may throw exceptions propagated
LWG 442 C++98 operator bool was not declared const (it is added const
const in the synopsis)
if os sets unitbuf, the destructor would
call os.flush(), which calls
LWG 835 C++98 is an UnformattedOutputFunction and creates os.rdbuf()->pubsync()
another sentry object in this case instead
(whose destructor then creates another
sentry object and so on)

See also

operator<< inserts formatted data
(public member function)