< cpp‎ | io‎ | basic istream
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 if skipws 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.


[edit] Member types

traits_type Traits

[edit] Member functions

constructs the sentry object. All the preparation tasks are done here
(public member function) [edit]
finalizes the stream object after formatted input or after exception, if necessary
(public member function) [edit]
not copy assignable
(public member function)
operator bool
checks if the preparation of the stream object was successful
(public member function) [edit]


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(failbit)(since c++11) 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() & 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(failbit | eofbit) (which may throw std::ios_base::failure).

Additional implementation-defined preparation may take place, which may call setstate(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.


is - input stream to prepare
noskipws - true if whitespace should not be skipped


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



Does nothing.

std::basic_istream::sentry::operator bool

explicit operator bool() const;

Checks whether the preparation of the input stream was successful.



Return value

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

[edit] Example

#include <iostream>
#include <sstream>
struct Foo
   char n[5];
std::istream& operator>>(std::istream& is, Foo& f)
    std::istream::sentry s(is);
    if (s), 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';



[edit] See also

extracts formatted data
(public member function) [edit]
extracts characters and character arrays
(function template) [edit]