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 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
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 subsequent 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_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.

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)