Return the length of the data in the overflow buffer, i.e., 0 if there is no overflow buffer, or dataLength() - initialBufferSize() if there is one.
dataLength() - initialBufferSize()
Declared in <bdlsb_overflowmemoutstreambuf.h>
<bdlsb_overflowmemoutstreambuf.h>
std::size_t dataLengthInOverflowBuffer() const;