How can I write a function template that can accept either a stack or a queue?

Question

I'm implementing four algorithms that are completely identical except for what data structure they use — two use priority_queue, one uses stack, and the last uses queue. They're relatively long, so I'd like to have just one function template that accepts the container type as a template argument and then have each algorithm call that template with the appropriate argument, like so:

template <class Container>
void foo(/* args */)
{
    Container dataStructure;
    // Algorithm goes here
}

void queueBased(/* args */)
{
    foo<queue<Item> >(/* args */);
}

void stackBased(/* args */)
{
    foo<stack<Item> >(/* args */);
}

I've managed to do just this with the priority_queue- and stack-based implementations, but I can't do the same for the queue-based algorithm because it uses a different name to access the foremost element (front( ) instead of top( )). I know that I could specialize the template for this case, but then I'd have a big stretch of duplicated code (which is what I'm trying to avoid).

What's the best way to accomplish this? My first instinct was to create a wrapper class for queue that adds a top( ) operation equivalent to stack's, but I've been reading that subclassing STL classes is a no-no. How should I get this behavior, then?

score 7 · Accepted Answer · answered Jan 27 '11 at 21:05

You can write a non-member top function overloaded on the type of the container adapter:

template <typename T>
T& top(std::stack<T>& s) { return s.top(); }

template <typename T>
T& top(std::queue<T>& q) { return q.front(); }

// etc.

If you actually use a different sequence container with the container adapters (via their Sequence template parameter), you'll need to modify the overloads appropriately to handle that.

It might just be more straightforward to use a sequence container (e.g. std::vector) directly rather than using one of the sequence adapters.

I'm implementing a set of search algorithms, so I need the specific ordering behavior of the adapters. (LIFO gives me a breadth-first search while FIFO gives me depth-first, e.g.) — Joel McCance, Jan 27 '11 at 21:17

score 2 · Answer 2 · answered Jan 27 '11 at 21:10

You can use partial specialization to select the right method:

template<class Container>
struct foo_detail {
  static typename Container::value_type& top(Container &c) { return c.top(); }
  static typename Container::value_type const& top(Container const &c) { return c.top(); }
};
template<class T, class Underlying>
struct foo_detail<std::queue<T, Underlying> > {
  typedef std::queue<T, Underlying> Container;
  static typename Container::value_type& top(Container &c) { return c.front(); }
  static typename Container::value_type const& top(Container const &c) { return c.front(); }
};

template<class Container>
void foo(/* args */)
{
    Container dataStructure;
    // Use foo_detail<Container>::top(dataStructure) instead of dataStructure.top().
    // Yes, I know it's ugly.  :(
}

score 1 · Answer 3 · answered Jan 27 '11 at 21:04

You can create a wrapper around std::queue without using inheritance; in fact, inheritance would be the wrong tool here because you're trying to decorate a queue rather than refining or extending the queue. Here's one possible implementation:

template <typename QueueType>
class QueueWrapper {
public:
    explicit QueueWrapper(const QueueType& q) : queue(q) {
        // Handled in initializer list
    }

    typedef typename QueueType::value_type value_type;

    value_type& top() {
        return queue.front();
    }
    const value_type& top() const {
        return queue.front();
    }

    void pop() {
        queue.pop();
    }
private:
    QueueType queue;
};

Hope this helps!

score 0 · Answer 4 · answered Jan 27 '11 at 21:03

queue, priority_queue and stack are all container adaptors; they are wrappers around an underlying container (by default, deque for queue and stack and vector for priority_queue).

Since vector, deque and list (the "real" container classes) share most of their methods, you could cut the middle man and use those classes instead.

And keep in mind that public inheritance is not a good idea for STL containers; private inheritance is okay (and probably what you want).

score -1 · Answer 5 · answered Jan 27 '11 at 20:57

-1

front() and top() are specific to certain types of containers, but all STL containers support *begin().

answered Jan 27 '11 at 20:57

aschepler

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`std::priority_queue`, `std::stack`, and `std::queue` are not containers and none of them are iterable sequences. – James McNellis Jan 27 '11 at 20:58

How can I write a function template that can accept either a stack or a queue?

5 Answers5