std::basic_istream<CharT,Traits>::sentry

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)
(destructor)
 finalizes the stream object after formatted input or after exception, if necessary
(public member function)
operator=
[deleted]
 not copy assignable
(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(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.

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

#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

See also

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

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