Overwriting virtual functions in body of code

Question

I have a question about virtual functions in C++. I have spent the last hour searching but I'm getting nowhere quickly and I was hoping that you could help.
I have a class that handles transmitting and receiving data. I would like the class to be as modular as possible and as such I would like to make an abstract/virtual method to handle the received messages.
Although I know I could create a new class and overwrite the virtual method, I don't really want to have to create a large array of new classes all implementing the method different ways. In Java you can use listeners and/or override abstract methods in the body of the code when declaring the object as can be seen in the example.

JTextField comp = new JTextField();   
comp.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent evt) {
        //Handler Code
    }
});

Is this possible in C++ or is there a better approach to this sort of problem?

Cheers and Many thanks in advance,
Chris.

The Java approach is a great way of creating write-only code. — Pete Becker, Aug 19 '13 at 12:56

score 3 · Accepted Answer · edited May 23 '17 at 10:25

3

Have look at this other SO post Does C++0x Support Anonymous Inner Classes as the question sounds similar.

Functors (function objects) or lambdas could be suitable alternatives.

edited May 23 '17 at 10:25

Community

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1

answered Aug 19 '13 at 12:01

Drew MacInnis

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That depends on how the function `addActionListener` works - if it stores an object derived from a base-class, then using lambdas or functors will not work. – Mats Petersson Aug 19 '13 at 12:04
Thanks Drew :), Big help. – Chris192 Aug 19 '13 at 12:23

score 1 · Answer 2 · answered Aug 19 '13 at 11:54

1

In C++, you need to declare a new class:

class MyActionListener: public ActionListener
{
    public:
       void actionPerformed(ActionEvent evt) { ... code goes here ... }
};

answered Aug 19 '13 at 11:54

Mats Petersson

126,704
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Or change the design to take some kind of polymorphic function object. – juanchopanza Aug 19 '13 at 12:01

score 0 · Answer 3 · answered Aug 19 '13 at 12:24

The question has already been answered but I thought I'd throw this in for good measure. The SO discussion linked in the example is good but concentrates mostly on replicating the Java experience. Here's a more idiomatic C++ way of doing it:

struct EventArgs
{
    int param1;
    int param2;
};

class network_io
{
    typedef std::function<void (EventArgs)> Event;
    typedef std::vector<Event> EventList;

    EventList Events;

public:
    void AddEventHandler(Event evt)
    {
        Events.push_back(evt);
    }

    void Process()
    {
        int i,j;
        i = j = 1;
        std::for_each(std::begin(Events), std::end(Events), [&](Event& e)
        {
            EventArgs args;
            args.param1 = ++i;
            args.param2 = j++;

            e(args);
        });
    }
};

int main() 
{
    network_io ni;

    ni.AddEventHandler([](EventArgs& e)
    {
        std::cout << "Param1: " << e.param1 << " Param2: " << e.param2 << "\n";
    });

    ni.AddEventHandler([](EventArgs& e)
    {
        std::cout << "The Param1: " << e.param1 << " The Param2: " << e.param2 << "\n";
    });

    ni.Process();
}

Overwriting virtual functions in body of code

3 Answers3