Why can't I make a priority queue of type struct objects?

debugcn Published at Dev

Viliami Mahe

So I kinda know that I can't make a priority queue with type struct but I don't exactly understand the reason why? I mean if you can create template classes to make your own type, why would a struct be any different?

This is what my code looks like in main.cpp:

#include <iostream>
#include <queue>
#include <string>

struct DATA {
    std::string key;
    int data;
};

int main() {
    std::priority_queue<DATA> priorityQ;
    DATA newItem;
    newItem.key = "apples";
    newItem.data = 3;
    priorityQ.push(newItem);
    std::cout << priorityQ.top().key << std::endl;

    std::cout << "Press the 'ENTER' key to continue...";
    std::cin.get();

    return 0;
}

The error come up with:

Error   C2678   binary '<': no operator found which takes a left-hand
operand of type 'const DATA' (or there is no acceptable conversion)
TestProj    c:\program files (x86)\microsoft visual studio 
14.0\vc\include\xstddef    Line: 240

I tried making this operator overload:

bool operator<(const DATA& a, const DATA& b) {
    return a.data > b.data;
}

But it still does not compile...

My question is: is it possible to put struct objects in priority queues and , and if not, why?

xaxxon

Here you go.

#include <iostream>
#include <queue>
#include <string>

struct DATA {
    std::string key;
    int data;
    // EITHER THIS
    bool operator<(const DATA & d2) const {return <your logic here>;}

};
// OR THIS
bool operator<(const DATA &d1, const DATA & d2){return <your logic here>;}


int main() {
    std::priority_queue<DATA> priorityQ;
    DATA newItem;
    newItem.key = "apples";
    newItem.data = 3;
    priorityQ.push(newItem);
    std::cout << priorityQ.top().key << std::endl;

    std::cout << "Press the 'ENTER' key to continue...";
    std::cin.get();

    return 0;
}

live: https://godbolt.org/g/uC6JdB

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