Priority queue with classes

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I am trying to use priority queue with custom classen. My class is node(name, .., .., distance, ...) but when in try to push a node into the queue i get a error.

node a = node("a",{}, 0, &a); node b = node("b",{}, INT_MAX, &a); node c = node("c",{}, INT_MAX, &a); node d = node("d", {}, INT_MAX, &a); node e = node("e", {}, INT_MAX, &a);
    
    edge a1 = edge(&a, &b,10); edge a2 = edge(&a, &c, 1); edge a3 = edge(&a, &d, 4);
    edge b1 = edge(&b, &d, 3); edge b2 = edge(&b, &e, 3);
    edge c1 = edge(&c, &d, 2);
    edge d1 = edge(&d, &e, 10); edge d2 = edge(&d, &b, 3);

    a.edges.push_back(&a1); a.edges.push_back(&a2); a.edges.push_back(&a3);
    b.edges.push_back(&b1); b.edges.push_back(&b2);
    c.edges.push_back(&c1);
    d.edges.push_back(&d1); d.edges.push_back(&d2);

    graph g = graph(
        {&a, &b, &c, &d, &e},
        {&a1,&a2,&a3,&b1,&b2,&c1,&d1,&d2}
    );

    node* start = &a;
    node* end = &e;


    std::vector<node*> eval = {start};
    std::vector<node*> test = {&a, &b, &c, &d};
    node* shortestNode;

    std::priority_queue<node> q;

    for(auto i : test)(
        q.push(i)
    )

What I have tried:

I have tried "copy paste" operator from the internet but none have been usefull.

The error i get is :

no instance of overloaded function "std::priority_queue<_Tp, _Sequence, _Compare>::push [with _Tp=node, _Sequence=std::vector<node, std::allocator<node>>, _Compare=std::less<node>]" matches the argument listC/C++(304)
main.cpp(37, 11): argument types are: (node *)
main.cpp(37, 11): object type is: std::priority_queue<node, std::vector<node, std::allocator<node>>, std::less<node>>

Posted 28-Jun-23 4:47am

Quinten Bakker (QB)

Updated 28-Jun-23 7:21am

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k5054 · Answer 1 · 2023-06-28T05:33:00

Similar to your previous question, a priority_queue needs to know how to determine which object has the higher priority. You could add a custom comparator to the definition of the priority_queue, or you could add an operator<() method to your node class.

C++

// priority_queue with custom comparator
auto comp = [](const node& a, const node& b){return a.distanceToStart < b.distanceToStart;};
std::priority_queue<node, vector<node>, decltype(comp)> pq(comp);

// add an operator<() to node:
class node {
 // ...
 public:
    bool operator<()( const node& other) const { return distanceToStart < other.distanceToStart );
}

I prefer adding the operator<() method to the class, but that's not possible if you only have a pre-compiled class definition. You might also use a custom comparator if the priority for the queue is different that a "default sort order".

Richard MacCutchan · Answer 2 · 2023-06-28T05:07:00

ObjectiveC

std::vector<node*> test = {&a, &b, &c, &d}; // test contains node pointers
node* shortestNode;

std::priority_queue<node> q; // priority_queue is declared to contain nodes rather than pointers to

for(auto i : test)(
    q.push(i) // trying to push a pointer into a container that wants an object
)

Rick York · Answer 3 · 2023-06-28T07:21:00

Another option is to implement a comparison method in your class specifically for the purpose of sorting. It has to be static since sort does not pass this pointer to it. You could also make a sort method that could also be static but necessarily. Those can look something like this :

C++

class node
{
public:
    using pnode  = node *;
    using vpnode = std::vector< pnode >;

    static bool CompareDistance( const pnode & lhs, const pnode & rhs )
    {
        return lhs->distanceToStart < rhs->distanceToStart;
    }

    static void SortNodes( vpnode & nodes )
    {
        std::sort( nodes.begin(), nodes.end(), CompareDistance );
    }
};

I find it helps clarity to have the using statements and a separate comparison method but opinions vary.

Priority queue with classes

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