How to add an remove std::functions from a std::vector

Question

I have a simple C++ program below which registers some callbacks/listeners which are just std::functions. And then it removes the callbacks.

Code:

#include <iostream>
#include <functional>
#include <vector>
#include <memory>

class SomeClass {
public:
    typedef std::function<void(int)> ListenerType;
    std::vector<ListenerType> m_listeners;

    void RegisterListener(const ListenerType& listener) {
       m_listeners.push_back(listener);
    }

    void UnregisterListener(const ListenerType& listener) {
        // This does not compile. Is this the right way to remove an std::function from va ector of  std::functions?
        auto position= std::find(m_listeners.begin(), m_listeners.end(), listener);
        if (position != m_listeners.end()) {
            m_listeners.erase(position);
        }
    }
};


class SomeOtherClass : public std::enable_shared_from_this<SomeOtherClass>{
public:
    SomeOtherClass(SomeClass some_class) : m_some_class(some_class) {   
    }

    void RegisterAllListeners() {
        m_some_class.RegisterListener(std::bind(&SomeOtherClass::ListenerMethod1, shared_from_this(), std::placeholders::_1));
        m_some_class.RegisterListener(std::bind(&SomeOtherClass::ListenerMethod2, shared_from_this(), std::placeholders::_1));
    }

    void UnregisterAllListeners() {
        m_some_class.UnregisterListener(std::bind(&SomeOtherClass::ListenerMethod1, shared_from_this(), std::placeholders::_1));
        m_some_class.UnregisterListener(std::bind(&SomeOtherClass::ListenerMethod2, shared_from_this(), std::placeholders::_1));
    }

private:
    SomeClass m_some_class;
    void ListenerMethod1(int value) {
        std::cout << "The value is: " << value << std::endl;
    }

    void ListenerMethod2(int value) {
        std::cout << "The value is: " << value << std::endl;
    }
};

int main() {
   SomeClass some_class;
   SomeOtherClass some_other_class(some_class);
   some_other_class.RegisterAllListeners();
   some_other_class.UnregisterAllListeners();
}

Question:
The issue is that UnregisterListener does not compile and fails with the following error.

error: overload resolution selected deleted operator '=='

But, this is exactly the way used to find and remove an item from a std::vector. Isnt it? What am I doing wrong?

Overall, my question is also to check whether this is the right way to add and remove listeners in C++ using std::functions?

Looking through this discussion, should I have to store the actual address of the function rather than the std:functions in the std::vector?

please include compiler errors in the question. – 463035818_is_not_an_ai Sep 08 '20 at 10:43 — 463035818_is_not_an_ai, Sep 08 '20 at 10:43
"_does not compile"_ What's the error? – Asteroids With Wings Sep 08 '20 at 10:44 — Asteroids With Wings, Sep 08 '20 at 10:44

Asteroids With Wings · Accepted Answer · 2020-09-08T10:51:17.967

2

You can't compare std::functions (except with nullptr), so that approach is never going to work (find cannot locate the function you're looking for).

Instead, wrap the std::function into some class that also has a unique ID for each one, then compare that instead (using a custom comparator for find).

Alternatively, and if you don't mind a bit of dynamic allocation, you could wrap the std::functions in std::shared_ptr<>, and store/pass those around to compare. However, be aware that two distinct (but "identical") functors would still compare false in this case, so you need to carefully think about what you're trying to achieve and how your code needs to be used.

Either way, ListenerType should be more than just an alias.

edited Sep 08 '20 at 10:51

answered Sep 08 '20 at 10:45

Asteroids With Wings

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"wrap the std::function into some class"? Is there an example I can follow? Or can you give a short example in the code I have? . Apparently, how to do this properly is also the objective of my question here? – TheWaterProgrammer Sep 08 '20 at 10:48
I'm sure your book teaches you how to write a class :) Indeed, you've already done so twice above! – Asteroids With Wings Sep 08 '20 at 10:49
1

Alright alright! I will figure it from the other question tagged. – TheWaterProgrammer Sep 08 '20 at 10:50
1

Very classy approach. – Hatted Rooster Sep 08 '20 at 12:29
@SombreroChicken Thanks! I typed it myself. – Asteroids With Wings Sep 08 '20 at 22:40

How to add an remove std::functions from a std::vector

1 Answers1