Question

I know that there are similar threads but after spending an hour trying to force my program to work, I decided to ask for a help.
First of all. I’ve thought that I know c++ pretty well since I tried something which is very simple in PHP(programming language which I know best) but very complexed in c++ (at least very complexed for me). So I want to create priority_queue of struct’s pointers. It’s obvious that I need to create my own compare function. So I tried this code:

#include <iostream>
#include <list>
#include <queue>

using namespace std;

typedef struct MI
{
    int nr;
    int koszt;
    bool operator<(const MI& a, const MI& b) {
      return a.koszt > b.koszt;
}
} miasto, *miasto_wsk;

int main()
{
    priority_queue<miasto_wsk> q;
    miasto_wsk mi;
    mi = new miasto;
    mi->nr = 1;
    mi->koszt = 2;
    q.push(mi);
}

And when I tried to compile my program I ended up with compilation error:

test.cpp:11:44: error: ‘bool MI::operator<(const MI&, const MI&)’ must take exactly one argument

Can you explain me what I’m doing wrong and explain me how all this stuff with structs compare works(or give me a good tutorial/article which explains that from the beginning)

EDIT:

I changed my code to this:

#include <iostream>
#include <list>
#include <queue>

using namespace std;

typedef struct miasto 
{
    int nr;
    int koszt;
} *miasto_wsk;

bool myComparator(miasto_wsk arg1, miasto_wsk arg2) {
      return arg1->koszt < arg2->koszt; //calls your operator
}

int main()
{
    priority_queue<miasto_wsk, vector<miasto_wsk>, myComparator> q;
    miasto_wsk mi;
    mi = new miasto;
    mi->nr = 1;
    mi->koszt = 2;
    q.push(mi);
}

And now I getting this error msg:

test.cpp: In function ‘int main()’:
test.cpp:19:64: error: type/value mismatch at argument 3 in template parameter list for ‘template<class _Tp, class _Sequence, class _Compare> class std::priority_queue’
test.cpp:19:64: error:   expected a type, got ‘myComparator’
test.cpp:19:67: error: invalid type in declaration before ‘;’ token
test.cpp:24:7: error: request for member ‘push’ in ‘q’, which is of non-class type ‘int’

What is the problem? Maybe I should use copies of structs instead pointers to structs?

EDIT2

This code doesn’t produce any compilation errors:

#include <iostream>
#include <list>
#include <queue>

using namespace std;

typedef struct miasto 
{
    int nr;
    int koszt;
    bool operator< (const miasto& rhs)
    {
    koszt > rhs.koszt;
    }
} *miasto_wsk;

int main()
{
    priority_queue<miasto_wsk> q;
    miasto_wsk mi;
    mi = new miasto;
    mi->nr = 1;
    mi->koszt = 22;
    q.push(mi);
}

So @Angew idea seems to be wrong.

EDIT3:
This is my final code. It not only compile without errors but also doing exactly what I want. Thank you so much @Angew

#include <iostream>
#include <list>
#include <queue>

using namespace std;

typedef struct miasto 
{
    int nr;
    int koszt;
} *miasto_wsk;

struct MyComparator {
    bool operator() (miasto_wsk arg1, miasto_wsk arg2) {
        return arg1->koszt > arg2->koszt; //calls your operator
    }
};


int main()
{
    //priority_queue<miasto_wsk, vector<miasto_wsk>, myComparator> q;
    priority_queue<miasto_wsk, vector<miasto_wsk>, MyComparator> q;
    miasto_wsk mi;
    mi = new miasto;
    mi->nr = 1;
    mi->koszt = 22;
    q.push(mi);
    miasto_wsk mi1;
    mi1 = new miasto;
    mi1->nr = 2;
    mi1->koszt = 50;
    q.push(mi1);
    miasto_wsk mi2;
    mi2 = new miasto;
    mi2->nr = 3;
    mi2->koszt = 1;
    q.push(mi2);

    cout << q.top()->koszt << endl;
    q.pop();
    cout << q.top()->koszt << endl;
    q.pop();
    cout << q.top()->koszt << endl;
    q.pop();
}

Answer 1

There are multiple issues here.

When you define an operator inside a class, it automatically takes a parameter of the class type as its first argument, and you must not create a parameter for it. So you either keep the operator in the class, like so:

struct MI {
  bool operator< (const MI&);
};

or declare the operator as free-standing:

struct MI {
  //...
};
bool operator< (const MI&, const MI&);

Second, your priority_queue stores pointers to MI, not instances of MI, so the operator will not be called anyway. You must provide a comparator when defining the priority queue, like this (EDITED):

struct MyComparator {
  bool operator() (miasto_wsk arg1, miasto_wsk arg2) {
    return *arg1 < *arg2; //calls your operator
  }
};

int main() {
  priority_queue<miasto_wsk, vector<miasto_wsk>, MyComparator> q;
  //...
}

Third is just a style thing: I’d suggest you name the class directly miasto rather than making it just a typedef. It’s more natural in C++.