Operator== overloading of an abstract class and shared_ptr

Question

I want to use std::find on a list of a shared_ptr of an abstract class, but I'm getting an error. Is there even a way to compare two shared_ptr by dereferencing them in std::find?

Is it possible to maybe make a friend operator== that overloads shared_ptr<A>?

Minimal example:

#include "point.h"
#include <list>
#include <algorithm>
#include <memory>

using namespace std;

class A {

protected:
    Point loc;
public:

    virtual void foo() = 0;

    virtual bool operator==(const Point& rhs) const = 0;
};

class B: public A {
    virtual void foo() override{}

    virtual bool operator==(const Point& rhs) const override {
        return rhs == loc;
    }
};

class C {

    list<shared_ptr<A>> l;
    void bar(Point & p) {

        const auto & f = find(l.begin(), l.end(), p); //<-- error is from here
    }
};

Error C2679 binary '==': no operator found which takes a right-hand operand of type 'const Point' (or there is no acceptable conversion)

Note: Point already has operator==.

Try defining `bool operator==(const std::shared_ptr &sp, const Point &point){return ((*sp) == point);}` — Rames, Jan 07 '16 at 19:20
@kuhaku have you declared it outside class? It cannot be a class member. — Rames, Jan 07 '16 at 19:28
Oh yeah it compiles now, why it can't be a member function? @Rames — shinzou, Jan 07 '16 at 19:31
@kuhaku because the lhs argument of member `operator==` is `this`, so it takes only one parameter more. Function I've written can be member of `std::shared_ptr` and then it would look like `bool std::shared_ptr ::operator==(const Point &point){return ((**this) == point);}`. However the solution mentioned by Christophe is generally better in terms of clean and 'good code', if I were you, I would implement that one. — Rames, Jan 07 '16 at 19:49

score 5 · Accepted Answer · edited Jun 20 '20 at 09:12

5

Problem:

find() is designed to find one precise value in an iterator range.

You have defined an operator== to compare an A with a Point. But your list doesn't contain A objects but shared pointers to A objects. Unfortunately comparing a shared pointer to a Point is not something that is defined. This mismatch causes the error that you've reported.

Solution:

An easy solution would be to use find_if() instead of find(): it doesn't look for a precise value, but for a predicate to become true:

   const auto & f = find_if(l.begin(), l.end(),[p](shared_ptr<A> &a){ return *a==p; });

edited Jun 20 '20 at 09:12

Community

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answered Jan 07 '16 at 19:21

Christophe

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It compiles. I knew lambda functions are cool, too bad our instructor dropped them, I'm gonna read up on them now. – shinzou Jan 07 '16 at 19:32
Yes, lambdas are definitively cool. Here a [quick intro](http://www.drdobbs.com/cpp/lambdas-in-c11/240168241) to start with. – Christophe Jan 07 '16 at 19:39

score 1 · Answer 2 · edited May 23 '17 at 12:23

1

std::find can be implemented as

template<class InputIt, class T>
InputIt find(InputIt first, InputIt last, const T& value)
{
    for (; first != last; ++first) {
        if (*first == value) {
            return first;
        }
    }
    return last;
}

As you can see it is comparing *first == value which is translates to a shared_ptr<A> == Point when using find(l.begin(), l.end(), p). Since it is going to use shared_ptr<A>::operator== you are going to use std::find_if and write a custom compare function/functor that can compare those two types and pass it to find

You can learn more about functors at: C++ Functors - and their uses

edited May 23 '17 at 12:23

Community

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answered Jan 07 '16 at 18:59

NathanOliver

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Or to pass special function to `find` – Slava Jan 07 '16 at 19:01
I have no clue about functors, does the special function for `find` have to be a functor? – shinzou Jan 07 '16 at 19:16
@kuhaku It doesn't have to be. You can make a normal function and pass it to `find` as well. Functors are suggested as they can be inlined which can make the code faster. – NathanOliver Jan 07 '16 at 19:17
As far as I know, compare function cannot be argument of `find`. http://www.cplusplus.com/reference/algorithm/find/ – Rames Jan 07 '16 at 19:22

Operator== overloading of an abstract class and shared_ptr

2 Answers2

Problem:

Solution: