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std::basic_istream::sentry::sentry

Prepares the stream for formatted input.

Ifis.good() isfalse, callsis.setstate(std::ios_base::failbit) and returns. Otherwise, ifis.tie() is not a null pointer, callsis.tie()->flush() to synchronize the output sequence with external streams. This call can be suppressed if the put area ofis.tie() is empty. The implementation may defer the call toflush() until a call ofis.rdbuf()->underflow() occurs. If no such call occurs before the sentry object is destroyed, it may be eliminated entirely.

Ifnoskipws is zero andis.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 inis). Ifis.rdbuf()->sbumpc() oris.rdbuf()->sgetc() returnstraits::eof(), the function callssetstate(std::ios_base::failbit|std::ios_base::eofbit) (which may throwstd::ios_base::failure).

Additional implementation-defined preparation may take place, which may callsetstate(std::ios_base::failbit) (which may throwstd::ios_base::failure).

If after preparation is completed,is.good()==true, then any subsequent calls tooperatorbool will returntrue.

Parameters

Exceptions

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

std::basic_istream::sentry::~sentry

Does nothing.

std::basic_istream::sentry::operator bool

Checks whether the preparation of the input stream was successful.

Parameters

(none)

Return value

true if the initialization of the input stream was successful,false otherwise.

#include <iostream>#include <sstream> struct Foo{char n[5];}; std::istream& operator>>(std::istream& is, Foo& f){    std::istream::sentry s(is);if(s)        is.read(f.n,5);return is;} int main(){std::string input="   abcde";std::istringstream stream(input);    Foo f;    stream>> f;std::cout.write(f.n,5);std::cout<<'\n';}

Output:

abcde