virtual std::streamsize showmanyc();
Estimates the number of characters available for input in the associated character sequence. underflow() is guaranteed not to return Traits::eof() until at least that many characters are extracted.
 Return value
The number of characters that are certainly available in the associated character sequence, or -1 if
showmanyc can determine, without blocking, that no characters are available. If
showmanyc returns -1, underflow() and uflow() will definitely return Traits::eof.
The base class version returns 0, which has the meaning of "unsure if there are characters available in the associated sequence".
|This section is incomplete
Reason: no example
 See also
| obtains the number of characters immediately available in the get area
(public member function)
| optionally provides the number of characters available for input from the file
(virtual protected member function of