Explanation of std::function

Question

What is the purpose of std::function? As far as I understand, std::function turns a function, functor, or lambda into a function object.

I

Answer 1

One example of where std::function can be very useful is in implementing an "observer pattern". So, for example, say you want to implement a simple "expression evaluator" calculator GUI. To give a somewhat abstract idea of the kind of code you might write against a GUI library using the observer pattern:

class ExprEvalForm : public GuiEditorGenerated::ExprEvalForm {
public:
    ExprEvalForm() {
        calculateButton.onClicked([] {
            auto exprStr = exprInputBox.get();
            auto value = ExprEvaluator::evaluate(exprStr);
            evalOutputLabel.set(std::to_string(value));
        });
    }
};

Now, how would the GUI library's button class store the function that's passed to onClicked? Here, an onClicked method (even if it were templated) would still need to store somewhere into a member variable, which needs to be of a predetermined type. That's exactly where the type erasure of std::function can come into play. So, a skeleton of the button class implementation might look like:

class PushButton : public Widget {
public:
    using ButtonClickedCallback = std::function;
    void onClicked(ButtonClickedCallback cb) {
        m_buttonClickedCallback = std::move(cb);
    }

protected:
    void mouseUpEvent(int x, int y) override {
        ...
        if (mouseWasInButtonArea(x, y))
            notifyClicked();
        ...
    }

private:
    void notifyClicked() {
        if (m_buttonClickedCallback)
            m_buttonClickedCallback();
    }
    ButtonClickedCallback m_buttonClickedCallback;
};