Question

If I have this class

class TextBox : public Widget {
public:
    void addKeyPressHandler(std::function<void (std::string)>);
private:
    std::function<void (std::string)> keyPressHandler;
};

How do I know if the keyPressHandler has been set or not? Can I initialize it to NULL and then test against that?

Answer 1

std::function allows you to test whether it holds a callable target via its operator bool().

For example:

if (keyPressHandler)
{
    keyPressHandler(someString);
}
else
    // TODO: Consider queuing events here if you don't want to miss keystrokes.
}

…will call the function only if it has a valid target. Meaning it has been assigned a function pointer, or anything else callable since it was initialized.