An object of class basic_istream::sentry is constructed in local scope at the beginning of each member function of std::basic_istream that performs input (both formatted and unformatted). Its constructor prepares the input stream: checks if the stream is already in a failed state, flushes the tie()'d output streams, skips leading whitespace unless noskipws flag is set, and performs other implementation-defined tasks if necessary. All cleanup, if necessary, is performed in the destructor, so that it is guaranteed to happen if exceptions are thrown during input.
Prepares the stream for formatted input.
If is.good() is false, calls is.setstate(std::ios_base::failbit) and returns. Otherwise, if is.tie() is not a null pointer, calls is.tie()->flush() to synchronize the output sequence with external streams. This call can be suppressed if the put area of is.tie() is empty. The implementation may defer the call to flush() until a call of is.rdbuf()->underflow() occurs. If no such call occurs before the sentry object is destroyed, it may be eliminated entirely.
If noskipws is zero and is.flags() & std::ios_base::skipws is nonzero, the function extracts and discards all whitespace characters until the next available character is not a whitespace character (as determined by the currently imbued locale in is). If is.rdbuf()->sbumpc() or is.rdbuf()->sgetc() returns traits::eof(), the function calls setstate(std::ios_base::failbit | std::ios_base::eofbit) (which may throw std::ios_base::failure).
Additional implementation-defined preparation may take place, which may call setstate(std::ios_base::failbit) (which may throw std::ios_base::failure).
If after preparation is completed, is.good() == true, then any subsequent calls to operator bool will return true.
Parameters is - input stream to prepare noskipws - true if whitespace should not be skipped Exceptionsstd::ios_base::failure if the end of file condition occurs when skipping whitespace.
std::basic_istream::sentry::~sentryDoes nothing.
std::basic_istream::sentry::operator boolexplicit operator bool() const;
Checks whether the preparation of the input stream was successful.
Parameters(none)
Return valuetrue if the initialization of the input stream was successful, false otherwise.
[edit] Example [edit] Defect reportsThe following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior LWG 195 C++98 it was unclear whether the constructor would seteofbit made clear LWG 419 C++98 the constructor did not set failbit if eofbit has been set sets failbit in this case [edit] See also
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