Problem 947

On WIN32-VC, the following coding error generates a General Protection Fault by attempting to dereference a null pointer within the STL::list<> class. This is a VERY flaky problem, and only appears occasionally for some simulations, and not at all for others.

File G4ExcitationHandler.cc (loop at line 215):

    std::list<G4Fragment*>::iterator j;
    for (j = theResultList.begin(); j != theResultList.end(); j++) {
        if( /* some test */) {
            // some code
            delete(*j);
            theResultList.erase(j--);
            // some code not using j
        } else {
            // some code that uses j
        }
    }

Clearly j can be decremented to point "previous" to begin(), which is undefined. Apparently on gcc it is OK (well, does not generate a fault, but I have no idea if it runs correctly). On Windows, VC++ generates a fault, at least if the list becomes empty from the erase().

This avoids the fault, BUT CHANGES THE LOGIC:
            // replace the above delete and erase() with:
            theResultList.erase(j);
            delete *j;
            if(theResultList.size() == 0) break;
            j--;

While that permits my simulation to execute, I believe it can omit generating some secondaries, so an expert on this process needs to look at it. I recommend changing that std::list<> to a std::vector<> and using an integer to index the vector -- an integer can be validly decremented to point before the start and then incremented before next use; an iterator cannot.

NOTE -- on WIN32-VC you must also implement the workaround to the compiler bug I reported in the "Installation and Configuration" forum thread 865.