std::basic_istream::sentry(3)

NAME
std::basic_istream::sentry - std::basic_istream::sentry

Synopsis
   class sentry;
   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.

Member types
   traits_type Traits

Member functions
   constructor constructs the sentry object. All the preparation tasks are done here
                 (public member function)
                 finalizes the stream object after formatted input or after exception,
   destructor if necessary
                 (public member function)
   operator= not copy assignable
   [deleted] (public member function)
   operator bool checks if the preparation of the stream object was successful
                 (public member function) std::basic_istream::sentry::sentry
   explicit sentry( std::basic_istream<CharT, Traits>& is, bool noskipws = false );
   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

Exceptions
   std::ios_base::failure if the end of file condition occurs when skipping whitespace. std::basic_istream::sentry::~sentry
   ~sentry();
   Does nothing. std::basic_istream::sentry::operator bool
   explicit operator bool() const;
   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.

Example
// Run this code
 #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
   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 195 C++98 it was unclear whether the constructor made clear
                      would set eofbit
   LWG 419 C++98 the constructor did not set failbit if sets failbit in this case
                      eofbit has been set

See also
   operator>> extracts formatted data
                        (public member function)
   operator>>(std::basic_istream) extracts characters and character arrays
                        (function template)