Priority queue of struct's pointers - c++

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++.

Answer 2

The error, if you read it again, tells you exactly what's wrong: That the MI::operator< function should take only one argument instead of two.
If you have operator< in the class (like you do) then the function takes only one argument and that is the other object to compare this with. If you create operator< as a free standing function (i.e. not part of the class) then it has to take two arguments.

Answer 3

Your comparison operator is a member function, so it should only take one parameter, for theRHS:
bool operator<(const MI& rhs) {
koszt > rhs.koszt;
}
Another option is to declare it as a non-member function:
struct MI {};
bool operator<(const MI& a, const MI& b) {
return a.koszt > b.koszt;
}

Answer 4

Use friend keyword to put the operator < in the global scope
typedef struct MI
{
int nr;
int koszt;
friend bool operator<(const MI& a, const MI& b)
{
return a.koszt > b.koszt;
}
} miasto, *miasto_wsk;