std::set - like function object support in my container

Question

I have implemented my own container:

template<typename T>
class MyContainer
{
    // body where in some point 2 elements of collection are compared (-1, 0 and 1 possible comparison results)
};

What I want to do is add support of function objects, just like in std::set, where it is possible to do function object like this:

struct Comparator
{
    bool operator()(const char* s1, const char* s2) const
    {
        return strcmp(s1, s2) < 0;
    }
};

and then pass it as set parameter:

std::set<const char*, Comparator> SomeSet;

I'm not every-day C++ programmer, so I need help to achieve that. What must I do in order to add support to this? Must I create field in MyContainer in order to store function object in it to use it in my sorting methods inside container?

Your thought of what to do is correct... have you tried it out? — David, Dec 01 '13 at 11:17
Usually it's done as functor template parameter with a default class given. — πάντα ῥεῖ, Dec 01 '13 at 11:17
One of my problems is that I don't know how to handle type-compatibility between type passed to container and operator () in Comparator. I'm not very good at templates. — Przemysław Kalita, Dec 01 '13 at 11:18
Just by calling the function. Compilation will fail if someone tries to pass a class that doesn't support the call operator. You could also add a static check for the `Comparator`class. — πάντα ῥεῖ, Dec 01 '13 at 11:20
Another problem is how to specify default Comparer if user did not pass anything? And how to properly compare 2 objects of unknown type in this comparer? — Przemysław Kalita, Dec 01 '13 at 11:29
@PrzemysławKalita: You can use the `std::less<>` template provided in the standard library. Look at e.g. `std::set`. It uses it too. — yzt, Dec 01 '13 at 11:43

score 0 · Accepted Answer · answered Dec 07 '13 at 13:44

I resolved it by adding default template value, and defining default comparing class:

template<typename T, class Compare = DefaultTreeOrder<T>>
class MyContainer
{
    private:
        Compare compare;
    public:
        MyContainer()
        {
            compare = Compare();
        }
};

where DefaultTreeOrder is:

template<typename T>
class DefaultTreeOrder 
{
    public:
        int operator()(T value1, T value2)
        {
            less<T> l;
            greater<T> g;

            if(l(value1, value2))
            {
               return 1;
            }
            else if(g(value1, value2))
            {
                return -1;
            }

            return 0;
        } 
};

std::set - like function object support in my container

1 Answers1