overloading operator outside of struct

Question

I have a struct and would like to implement a set of its pointers. So I try to overload operator < to make it happen. One limitation here is that I do not have access to write overloading code within the struct definition. How can I do it outside of the struct?

Here is what I have so far:

#include<iostream>
#include<set>
#include<vector>

using namespace std;

struct mystruct {
    int label;
    vector<mystruct *> neighbors;
};
bool operator < (mystruct * n1, mystruct* n2) {
    return n1 -> label < n2 -> label;
};
int main() {
    set<mystruct *> s;
    return 0;
}

The error message is

error: overloaded 'operator<' must have at least one parameter of class or enumeration type

Wh do you need pointers in your set? You can't overload intrinsic operators. — πάντα ῥεῖ, May 25 '17 at 16:19
@πάνταῥεῖ I mean I am not allowed to define operator < in the struct scope. — Yi Bao, May 25 '17 at 16:23
But why do you need pointers in the set? You can't overload the definition of the `operator<()` for pointers. — πάντα ῥεῖ, May 25 '17 at 16:25
@πάνταῥεῖ I changed the question into "implement a set of its pointers". And if it is set, the code works. — Yi Bao, May 25 '17 at 16:29
`int main() { set s; {mystruct m; s.insert(&m);} return 0;}` I have just broken your `set`, since it now has a bogus pointer as an element. That's why you're being asked why you need a set of pointers. — PaulMcKenzie, May 25 '17 at 16:30
_"And if it is set, the code works."_ Sure. Why did you think of pointers in 1st place? What's the problem (use case) you want to solve? — πάντα ῥεῖ, May 25 '17 at 16:30
@πάνταῥεῖ Good question. I am actually dealing with a problem related to graph. Structs have neighbors. I edited the code again with one more detail. Now struct has a vector of pointers, which is given by the problem. — Yi Bao, May 25 '17 at 16:41
@YiBao I'm not so sure if `std::set` is the correct data structure to solve your original problem. — πάντα ῥεῖ, May 25 '17 at 16:43
Keen observation. It turns out `std::set` is not a good choice. — Yi Bao, May 25 '17 at 18:53

NathanOliver · Accepted Answer · 2017-05-25T16:45:33.680

3

The problem with

bool operator < (mystruct * n1, mystruct* n2) {
    return n1 -> label < n2 -> label;
};

Is that both n1 and n2 are pointers. Even though they are pointers to mystruct they are still just pointers and you cannot overload operators for pointers as they are built in. The simplest way to fix this is to take references instead and work with values like

bool operator < (const mystruct& n1, const mystruct7 n2) {
    return n.label < n2.label;
};

int main() {
    set<mystruct> s;
    return 0;
}

If you can't do that then you need to provide a comparison functor to the std::set and have it use that function instead of operator <. That would look like

struct mystruct_pointer_comp
{
    bool operator ()(mystruct * n1, mystruct* n2) {
        return n1->label < n2->label;
    };
}

int main() {
    set<mystruct *, mystruct_pointer_comp> s;
    return 0;
}

edited May 25 '17 at 16:45

answered May 25 '17 at 16:25

NathanOliver

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An intermediate solution (if you don't want to give ownership of the objects to the container) could be to create the "regular" overload with references, and use a `std::set>`. – Matteo Italia May 25 '17 at 16:34
@MatteoItalia Not sure how well that would work since the objects the wrapper wraps will need to exist outside the set and for as long as the set exists. – NathanOliver May 25 '17 at 16:36
@MatteoItalia Most probably more something like a `std::set>` then. But it's questionable why to use a `std::set` then at all. – πάντα ῥεῖ May 25 '17 at 16:41

overloading operator outside of struct

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