std::vector of std::functions find

Question

I have a vector populated with callback functions and I would like to check whether callback to the function already exists prior to adding it. I don't know whether it will even work bu so far it doesn't even compile.

vector<std::function<void(void*)>> _callbacks;

void Event::RegisterCallback(std::function<void(void*)> callback)
{
    if (callback == NULL)
        return;

    vector<std::function<void(void*)>>::iterator it = std::find(_callbacks.begin(), _callbacks.end(), callback);

    if (it == _callbacks.end())
    {
        _callbacks.push_back(callback);
    }
    else
    {
        //print error
        throw;
    }
}

This gives a compile error: "Overload resolution selected deleted operator '=='" in alorithm(805). This is related to the find function call.

How do I get this to work and is it even going to compare function calls to the same method properly?

Thanks

Answer 1

As noted in the comments the simplest solution is to use default C-style function pointers as they support == operator in opposite to C++11 function which does not.

using func_type = void(*)();
vector<func_type> _callbacks;

void Event::RegisterCallback(func_type callback)
{
    if (callback == nullptr)
       return;

    auto it = std::find(_callbacks.begin(), _callbacks.end(), callback);

    if (it == _callbacks.end()) {
        _callbacks.push_back(callback);
    }
    else {
        throw;
    }
}

void f() {};
void g() {};

/*
    evt.RegisterCallback(f); // works fine
    evt.RegisterCallback(g); // works fine
    evt.RegisterCallback(f); // throws exception
*/

If you don't like this approach you can write your own function-pointer class with support of equality operator.