C++11 alternative to the Java anonymous callback class

Question

I realise that the solution I have here is far from ideal with C++, so I'm asking what a proper C++ programmer would do in this situation. (C++11)

I have a DialogBox class, which stores a collection of buttons. At the moment I have a pure abstract inner class DialogBox::Button with the pure virtual function virtual void callback() const.

From Java I'm used to using this strategy to create and instantiate an anonymous class deriving from Button in-place which implements the callback function. Something like this:

db.add_button(new Button( "Button text", BUTTONTYPE ) {
    public void callback() {
        // Callback code
}});

which is what prompted this C++ solution.

My C++ solution therefore looks like

dialogbox.h

class DialogBox {
public:
    // Abstract class for buttons with callback functions
    class Button;

private:
    /* ...
      stuff 
     */

public:
    /* ...
      stuff 
     */
    const std::vector< unique_ptr<DialogBox::Button> >& get_buttons() const;
    void add_button( unique_ptr<DialogBox::Button>& new_button );
};


class DialogBox::Button {

private:
    /* ...
      stuff
     */

public:
    // Constructor
    Button( const string& button_text, const DialogButtonType button_type = DialogButtonType::NORMAL );

    /* ...
      stuff
     */

    // Virtual callback function
    virtual void callback() const = 0;

};

Usage:

// Instantiate
DialogBox db{ /* ... args ... */ };
// Test adding buttons
class Button1 : public DialogBox::Button {
    using DialogBox::Button::Button;
    public: void callback() const {
        // Callback code
    }
};
std::unique_ptr<DialogBox::Button> button1{ new Button1{ "Button1", DialogButtonType::ENTER } };
db.add_button( button1 );

This works, but it's clearly not as clean as the Java version and certainly feels like I'm shoehorning in something that C++ is not designed to do.

So, how would a C++ programmer do this? It seems conceptually right to have Button as a class (since it has internal data and its own behaviour). At the moment I'm thinking of using a lambda expression to pass in the callback function to Button's constructor, but I thought I'd get some expert opinion on the subject.

I realise this, which is why I've come in search of good-practice solutions. — JMAA
– JMAA, Commented May 30, 2015 at 12:01
@pathfinderelite yes, it's C++11. Do you suggest I use lambdas as described in my last paragraph? — JMAA
– JMAA, Commented May 30, 2015 at 12:12

Barry · Accepted Answer · 2015-05-30 12:42:18Z

4

The C++11 solution would be to have Button look something like this. I'm skipping the string and DialogButtonType parameters for brevity:

class Button {
public:
    template <typename F>
    Button(F&& f) : cb(std::forward<F>(f))
    { }

    void callback() { cb(); }

private:
    std::function<void()> cb; // type-erased functor for ANY callable
                              // that takes zero arguments
};

This allows you to have a container of Buttons that do completely arbitrary things in their callbacks - no inheritance necessary. It also allows you to great one-off buttons on the fly by providing them with an arbitrary callback functor as part of the construction process:

db.add_button(std::make_unique<Button>([=]{
     // arbitrary Callback code
}));

As a side-note, add_button should definitely take its unique_ptr argument by value, not by reference.

answered May 30, 2015 at 12:42

Barry

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4 Comments

JMAA Over a year ago

Thanks, this looks good. Should I also be returning the vector of unique_ptrs by value rather than by reference?

Barry Over a year ago

@JMAA By reference. You'd have to move it out otherwise - unless that's the semantic you want, in which case, sure, by value.

Yakk - Adam Nevraumont Over a year ago

Why have a class Button at all? It seems useless. typedef std::function<void()> Button -- it isn't really a button, just an action to take when clicked.

Barry Over a year ago

@Yakk I assume the name had some significance for something?

Yakk - Adam Nevraumont · Accepted Answer · 2015-05-31 13:57:54Z

0

Consider an aggregate:

struct Button {
  std::string name;
  std::function<void()> on_click;
};

now you can add_button({"hello", []{std::cout<<" world\n";}});

Minimal code, minimal boilerplate.

I generally start with this kind of thing, and only add more infrastructure as needed.

answered May 31, 2015 at 13:57

Yakk - Adam Nevraumont

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2 Comments

JMAA Over a year ago

This looks nicely minimal. Sorry for the noob question, but does it allow me to deal with const buttons and other access controls?

Yakk - Adam Nevraumont Over a year ago

@jmaa The button is taken by-value, and its constness means you cannot change the action/name, but not much else.

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