Problem with c++ sorting of objects, vectors

Question

Consider the following code:

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

class Vertex {
    public:
    int id;
    vector<Vertex*> edges;
    Vertex(int id) : id(id) {}
    int get_id() const {return id;}
};

class Graph {
    public:
    vector<Vertex*> vertices;
    Graph(int V) {
        vertices.reserve(V);
    }
    void test(vector<Vertex*>& other) {
        sort(vertices.begin(), vertices.end(), [](const Vertex*& one, const Vertex*& two) {return &*one < &*two;});
        sort(other.begin(), other.end(), [](const Vertex*& one, const Vertex*& two) {return &*one < &*two;});
    }
};

When I try to compile the above I get the error: error: no matching function for call to object of type '(lambda at Graph.cpp:59:48)' if (__comp(*--__last, *__first)). I don't understand how I can fix this issue.

@sweenish but I don't compare any vertex objects? I only compare their memory addresses. — Jacob
– Jacob, Commented Mar 30, 2021 at 18:24
Why are you comparing adresses in your lambdas? Also why add the reference the pointers; you're not saving any space by using a reference to a pointer instead of a pointer. — fabian
– fabian, Commented Mar 30, 2021 at 18:25
@Jacob so I can check whether the two vectors have any pointers that point to the same vertex object -- Then that begs the question -- what do you do if you found out there are pointers pointing to the same vertex? In all honesty, your code looks like an XY problem scenario. — PaulMcKenzie
– PaulMcKenzie, Commented Mar 30, 2021 at 18:38

HolyBlackCat · Accepted Answer · 2021-03-30 18:26:25Z

3

The parameters of your comparators are non-const references. More specifically, "references to non-const pointers to const Vertex".

You either need const references: const Vertex *const &one, or even better, just pass by value: const Vertex *one.

Also note that &*one < &*two is equivalent to one < two.

edited Mar 30, 2021 at 18:26

answered Mar 30, 2021 at 18:22

HolyBlackCat

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2 Comments

HolyBlackCat Over a year ago

@Jacob You're welcome! You can press the checkmark on the left to mark your question as solved.

Jacob Over a year ago

I'm aware of this, but one can only do this after a certain amount of minutes.

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