[BGL] setting a breakpoint in an algorithm visitor

p.156 of the BGL book: "... visitors are passed by value..." I was attempting to set a breakpoint in a visitor event handler (that I know is executing) but the breakpoint is seemingly ignored by my debugger. This is because the visitor I construct and subsequently pass to the algorithm is not the object that is actually dispatched by the algorithm. Correct? Using the Intel compiler under MS DevStudio 6 on Windows, I inserted a ::DebugBreak() (asm int 3) into the visitor and *do* hit that but only have assembly language and not source to step through at that point. Why are visitors passed to algorithms by value? Any suggestions for debugging visitor code at the C++ source level and not in assembly? - Regards Chris