# C++ concepts: UnformattedInputFunction

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### Requirements

An `UnformattedInputFunction` is a stream input function that performs the following:

- Constructs an object of type `basic_istream::sentry` with automatic storage duration and with the `noskipws` argument set to true, which performs the following

  - if `eofbit` or `badbit` are set on the input stream, sets the `failbit` as well, and if exceptions on failbit are enabled in this input stream's exception mask, throws `ios_base::failure`.
  - flushes the tie()'d output stream, if applicable

- Checks the status of the sentry by calling `sentry::operator bool()`, which is equivalent to `basic_ios::good`.
- If the sentry returned false or sentry's constructor threw an exception:

  - sets the number of extracted characters (gcount) in the input stream to zero
  - if the function was called to write to an array of `CharT`, writes `CharT()` (the null character) to the first location of the array

- If the sentry returned true, performs the input as if by calling rdbuf()->sbumpc() or rdbuf()->sgetc().

  - if the end of the stream is reached (the call to rdbuf()->sbumpc() or rdbuf()->sgetc() returns Traits::eof(), sets `eofbit`. If exceptions on eofbit are enabled in this stream's exception mask, throws `ios_base::failure`.
  - if an exception is thrown during input, sets `badbit` in the input stream. If exceptions on badbit are enabled in this stream's exception mask, the exception is also rethrown.
  - If no exception was thrown during input, sets the number of extracted characters (gcount) in the input stream.

- In any event, whether terminating by exception or returning, the sentry's destructor is called before leaving this function.

### Standard library

The following standard library functions are `UnformattedInputFunction`s.

- std::getline, except that it does not modify gcount.
- `basic_istream::operator>>(basic_streambuf*)`
- `basic_istream::get`
- `basic_istream::getline`
- `basic_istream::ignore`
- `basic_istream::peek`
- `basic_istream::read`
- `basic_istream::readsome`
- `basic_istream::putback`, except that it first clears `eofbit`
- `basic_istream::unget`, except that it first clears `eofbit`
- `basic_istream::sync`, except that it does not modify gcount
- `basic_istream::tellg`, except that it does not modify gcount
- `basic_istream::seekg`, except that it first clears `eofbit` and does not modify gcount
- std::ws, except that it does not modify gcount

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