C++ : struct vs function for ordering elements - c++

Question

I have a struct with two fields :
struct road {
int from, len ;
};
For some reason, I need to be able to order my roads :
by ascending from in an array
by ascending len in a priority queue
I have thus included :
#include <iostream>
#include <algorithm>
#include <queue>
#include <vector>
I have come across websites suggesting to overload the operator<, but because of the two possible orderings that just feels wrong and it would only solve one of the two.
By messing around with textbooks, I got this to work :
bool cmpFrom (const road & a, const road & b) {
return (a.from < b.from) ;
}
struct cmpLen {
bool operator () (const road & a, const road & b){
return (a.len < b.len) ;
}
};
To be used with :
std::sort(trips, trips + nbRoads, &cmpFrom) ;
std::priority_queue<road, std::vector<road>, cmpLen> pickRoad ;
Where trips is of course a road [].
It compiles perfectly (haven't tried running it, but it should be fine), but it seems weird to define two very similar comparators in two quite different manners, so isn't there a way to define both comparison methods the same way ?
Changing the definition of cmpFrom to
struct cmpFrom {
bool operator () (const road & a, const road & b){
return (a.from < b.from) ;
}
};
Gives
chantier.cpp: In function ‘int main()’:
chantier.cpp:38:48: error: expected primary-expression before ‘)’ token
std::sort(trips, trips + nbRoads, &cmpFrom) ;
Which I assume means "You gave me a type when I was expecting a reference".
While writing
bool cmpLen (const road & a, const road & b) {
return (a.len <= b.len) ;
}
Gives
chantier.cpp: In function ‘int main()’:
chantier.cpp:52:56: error: type/value mismatch at argument 3 in template parameter list for ‘template<class _Tp, class _Sequence, class _Compare> class std::priority_queue’
std::priority_queue<road, std::vector<road>, cmpLen> pickRoad ;
^
chantier.cpp:52:56: note: expected a type, got ‘cmpLen’
chantier.cpp:56:30: error: request for member ‘top’ in ‘pickRoad’, which is of non-class type ‘int’
...
Is there a way to make one of these comparison methods work for both containers ? Or is there perhaps a third way of doing this that could work with both ?
What if I had needed to use the same ordering with both containers ? Would that have required defining twice the same comparison method, but with one inside a struct ?

Answer 1

You almost have it. In std::sort you need an object that you can call operator() on. Using
bool cmpFrom (const road & a, const road & b) {
return (a.from < b.from) ;
}
std::sort(trips, trips + nbRoads, &cmpFrom);
works because a function pointer can be used like a function. When you change cmpFrom to
struct cmpFrom {
bool operator () (const road & a, const road & b){
return (a.from < b.from) ;
}
};
you can't use std::sort(trips, trips + nbRoads, &cmpFrom); anymore because you can't apply & to a type name. Instead what you need to do is get an object of cmpFrom and you do that like
std::sort(trips, trips + nbRoads, cmpFrom{});
now both the priority_queue and sort could use cmpFrom.

Answer 2

It's easier to define both as structures, because you can always create an object from a type and it will behave as expected, but getting a type from a function and having it act as a caller for the function is much more difficult.
You were in fact almost there with struct cmpFrom. However, you've correctly noted that std::sort expects a comparator object (such as a function), not a type. Of course, doing &cmpFrom where cmpFrom is a type is not valid C++. Instead, you need to create an object of that type; thanks to the operator() defined, the object will be callable and do what you want. So just call std::sort like this:
std::sort(trips, trips + nbRoads, cmpFrom{});

Answer 3

The std::sort function and std::priority_queue class template want two different things: sort wants a callable object, while priority_queue template wants a type, which allows creating objects.
Because of that, sort is more omnivorous than priority_queue - you can use it with either functions or functors. The only thing you need is to provide it with a real object (while currently in your code you are trying to take an address of a type, which makes no sense).
To fix it in your example just change the code to
std::sort(trips, trips + nbRoads, cmpFrom{});