cpp/io/basic streambuf/showmanyc

Estimates the number of characters available for input in the associated character sequence. underflow is guaranteed not to return or throw an exception until at least that many characters are extracted.

Parameters
(none)

Return value
The number of characters that are certainly available in the associated character sequence, or if  can determine, without blocking, that no characters are available. If returns, underflow and uflow will definitely return  or throw.

The base class version returns, which has the meaning of "unsure if there are characters available in the associated sequence".