cpp/locale/codecvt/length

@1@ Public member function, calls the member function of the most derived class.

@2@ Attempts to convert the characters from the character array defined by, given initial conversion state , to at most   characters, and returns the number of  characters that such conversion would consume. Modifies as if by executing  for some imaginary  output buffer.

Return value
The number of characters that would be consumed if converted by do_in until either all  characters were consumed or   characters were produced, or a conversion error occurred.

The non-converting specialization returns.