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std::basic_istream::sentry::sentry

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

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