how to enable trace in bison C++ parser

[Gvnaena Pura]

Hi,

I know for C parsers, one can simply #define YYDEBUG to be 1, and set
yydebug=1 somewhere, and then the parser will print trace information
at each step.  But how can I do this for C++ parser?  I
used ::set_debug_level and ::set_debug_stream, and there is no effect
at all.  How can I do this?  Thanks in advance.