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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 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 397 C++98 the destructor might call os.flush(), which the exception is not may throw exceptions prop- agated 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) http://cppreference.com 2024.06.10 std::basic_ostream::sentry(3)
NAME | Synopsis | Member functions | Parameters | Exceptions | Parameters | Return value | Example | Output: | See also
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