Question

In my current C++-project I have an STL map which maps integer keys onto objects. An algorithm returns a set of entries. The returned data depends on the algorithm’s input and hence cannot be predicted:

  class MyClass
  {
     //...
  };

  int myAlgorithm(vector<int>::iterator inputIt)
  {
     // return a key for myMap which is calculated by the current value of inputData
  }

  int main(int argc, char *argv[])
  {
     vector<int> inputData;
     map<int, MyClass> myMap;
     //<fill map with some data>
     //<fill inputData>

     vector<MyClass> result;

     for (vector<int>::iterator it = inputData.begin(); it != inputData.end(); it++)
     {
        int myMapKey = myAlgorithm(*it);
        // count() > 0 means "check whether element exists. Performance can be improved by replacing
        //    the operator[] and count() calls by map::find(). However, I want to simplify things
        //    in this example.
        if (myMap.count(myMapKey) > 0)
        {
           // in some cases there is no entry in myMap
           result.push_back(myMap[myMapKey]);
        }
     }
  }

As mentioned in the example I can replace map::count() and operator[]-calls with find. The STL-reference says that map::find()’s complexity is logarithmic in size (O(log n)).

I discovered that in most cases the entries in myMap are very close for two sequent entries in the result. Therefore I came to the conclusion that I would achieve better performance if I replaced the map.find() calls by iterators:

     map<int, MyClass>::iterator myMapIt = myMap.begin();
     for (vector<int>::iterator it = inputData.begin(); it != inputData.end(); it++)
     {
        int myMapKey = myAlgorithm(*it);
        // just increment iterator
        while (myMapKey != myMapIt->first)
        {
           myMapIt++;
           // we didn't find anything for the current input data
           if (myMapIt == myMap::end() || myMapIt->first > myMapKey)
           {
              break;
           }
        }

        // I know that I'm checking this twice, but that's not the point of my
        //    question ;)
        if (myMapIt == myMap::end() || myMapIt->first > myMapKey)
        {
           // probably it would be better to move the iterator back to the position
           //    where we started searching, to improve performance for the next entry
           myMapIt = myMap.begin();
        }
        else
        {
           result.push_back(myMapIt.second);
        }
     }

This concept works but I have a big problem: Depending on the inputData, I have to search forward or backward. Consider that I call the code inside main() multiple times and the inputData changes for these calls. Instead of checking whether to increment or decrement the iterator inside the while-loop, I could decide that before entering the for-loop.

I thought that I’m fine with just switching the map<>::iterator to map<>::reverse_iterator and using rbegin()/rend() instead of begin()/end(). But then I realized that reverse_iterator and iterator have no common base class:

     map<int, MyClass>::base_iterator myIt;
     if (/* ... */)
     {
        myMapIt = myMap::begin();
        myMapEndIt = myMap::end();
     }
     else
     {
        myMapIt = myMap::rbegin();
        myMapEndIt = myMap::rend();
     }
     /* for (...) ... */

That would be great, but there is no base_iterator.

I know a simple workaround for this problem: I just need to copy the whole for-loop and adjust it for both cases:

     if (/* ... */)
     {
        /* for(...) which uses normal iterator in the while-loop */
     }
     else
     {
        /* for(...) which uses reverse iterator in the while-loop */
     }

Very bad … Do you know a better solution?

Answer 1

A common base type is unnecessary when the language allows Generic Programming.

What you simply need to realize is that instead of having a long-winded linear functions with several choices along the way, you can have several nested function in which each choice lead to a different call.

Taking your example:

boost::any_iterator start, end;
if (/* ... */) {
  start = map.begin(), end = map.end();
} else {
  start = map.rbegin(), end = map.rend();
}

// do something with start and end

You can transform the code into the following:

// Define a free-function in the .cpp to help factor common stuff
template <typename FwdIt>
static void dosomething(FwdIt start, FwdIt end) {
  // do something with start and end
}

And then inject the call straight into the if/else body:

if (/* ... */) {
  dosomething(map.begin(), map.end());
} else {
  dosomething(map.rbegin(), map.rend());
}

And one good thing is that you reduce the number of changes of states within your functions and thus their complexity.