Question

starting with priority_queues, I have a problem like this: I need elements to be stored in a queue, but the criterion how they are sorted is not included in the elements itself, but somewhere different, like in a map:

std::map<element, value> element_values;
std::priority_queue<element> queue;

What I need now is something like that:

struct Comp
{
    std::map<...>& the_map;
    Cpmp(std::map<...> _map) : the_map(_map) {}

    bool operator() (element a, element b)
    {
        return the_map[a] < the_map[b];
    }
 }

 Comp comp(element_values);
 std::priority_queue<element, std::vector<element>, comp> queue; // does not work
 std::priority_queue<element, std::vector<element>, Comp> queue; // does work but I'd not be able to pass values to the constructor

The elements itself have no intrensic order. A workaround would be to define a struct wrapping this stuff up, but maybe someone knows a smarter way. I also thought about providing a compare function only valid in my current scope (which itself is a function), but as far as I know C++ does not support that, at least not with capturing local variables like I would need.

Answer 1

The std::priority_queue<T, Cont, Comp> take the comparison object type as template argument. To pass an object referencing something, you need to pass it as constructor argument:

std::priority_queue<element, std::vector<element>, Comp> queue(comp);