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