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)