stl priority_queue of C++ with struct

Question

How can we use STL priority_queue for struct ? Any illustration of pushing & popping , where struct has multiple data-types?
Say : struct thing { int a; char b;} glass[10]; .
Now how can i put this struct on priority_queue using 'int a' for ordering ?

Answer 1

Priority Queue in Structure can be Implemented in two ways::

1. Overloading the < operator
2. Passing the boolean comparator operator wrapped in structure.

Let us take a structure car which has model and price both integers and build priority queue based on highest price at the top.

struct Car
{
    int m;
    int p;
    void set(int a,int b){
        m =a,p = b;
    }
};

For Method 1 we need to do this:

bool operator < (const Car& c1,const Car& c2){
    return c1.p < c2.p;
}

For Method 2 we need to pass this structure:

struct Cmp{
    bool operator()(const Car& c1,const Car& c2){
            return c1.p < c2.p;
    }
};

Full Code for illustration:::

#include <iostream>
#include<unordered_map>
#include<string>
#include<queue>
#include<vector>
using namespace std;

struct Car
{
    int m;
    int p;
    void set(int a,int b){
        m =a,p = b;
    }
};
struct Car cT[50];
int cI=0;
void printC(){
    for(int i=0;i<cI;i++){
        cout <<" car model: " << cT[i].m << " price: " <<cT[i].p <<endl;
    }
}

bool operator < (const Car& c1,const Car& c2){
    return c1.p < c2.p;
} //Method 1

struct Cmp{
    bool operator()(const Car& c1,const Car& c2){
            return c1.p < c2.p;
    }
}; //Method 2

void pushQ(priority_queue<Car,vector<Car>,Cmp>& Q){
 for(int i=0;i<cI;i++){
        Q.push(cT[i]);
        cout <<"Queue Push car model: " << cT[i].m << " price: " <<cT[i].p <<endl;
    } 
};

void printQ(priority_queue<Car,vector<Car>,Cmp> Q){
    while(!Q.empty()){
        Car c = Q.top();
        Q.pop();
        cout <<" car model: " << c.m << " price: " <<c.p <<endl;
    }
}
int main() {
    // Write C++ code here
    
    // priority_queue<Car> Q; // #Method 1 
    priority_queue<Car,vector<Car>,Cmp> Q;// #Method 2 
    cT[cI++].set(4,5);
    cT[cI++].set(34,4);
    cT[cI++].set(43,6);
    cT[cI++].set(41,15);
    pushQ(Q);
    printQ(Q);
   // printC();
    // cout << "Hello world!" <<f1;

    return 0;
}

Output will be:::

car model: 41 price: 15
car model: 43 price: 6
car model: 4 price: 5
car model: 34 price: 4

Answer 2

You can do it like this!

struct example{
   int height;
   int weight;
};

struct comp{
        bool operator()(struct example a, struct example b){
         //Sorting on the basis of height(Just for example)
            return (a.height > b.height);
        }
    };

// And here comes your priority queue
 priority_queue<struct example, vector<struct example>, comp> pq;
struct example your_obj;
pq.push(your_obj);

Answer 3

You need to implement a compare function or overload operator to tell priority queue that on which order you want to sort your custom data. When priority queue will sort your data then it will need a way to know how to compare among them. You have to specify this by passing a function to priority queue or overloading operator in you custom data class or structure.

You can check this answer. This might help you. I have tried to explain multiple ways of using priority queue for custom data types.

Answer 4

Here is a slightly modified answer to your original question, which you deleted for no apparent reason. The original contained enough information for you to figure this out, but here it goes: provide a less than comparison that uses the int for comparison.

All you need to do is provide a functor that implements a less-than comparison with strict weak ordering, or a less-than operator for your class implementing the same. This struct satisfies the requirements:

struct thing
{
    int a;
    char b;
    bool operator<(const thing& rhs) const
    {
        return a < rhs.a;
    }
};

then

std::priority_queue<thing> q;
thing stuff = {42, 'x'};
q.push(stuff);
q.push(thing{4242, 'y'}); // C++11 only
q.emplace(424242, 'z'); // C++11 only    
thing otherStuff = q.top();
q.pop();

Answer 5

Overload < operator for thing :

struct thing
{
    int a;
    char b;

    bool operator<(const thing &o) const
    {
        return a < o.a;
    }
};

priority_queue<thing> pq;

thing t1, t2, t3;

// ...

pq.push(t1);
pq.push(t2);

// ...

t3 = pq.top();
pq.pop();