Question

In the class below, would this mean that onPaintCallback is NULL, or must I make it NULL in the class constructor? I want to start checking for NULL before it is given a valid pointer.

class AguiWidgetBase
{
    virtual void onPaint();
    void (*onPaintCallback)(AguiRectangle clientRect) = 0;
public:
    AguiWidgetBase(void);
    ~AguiWidgetBase(void);
};

Answer 1

What you have isn’t legal. You have to initialize it in the constructor:

AguiWidgetBase::AguiWidgetBase() :
onPaintCallback(0)
{}

You could use boost::function<void(AguiRectangle)>, which aside from being more flexible, initializes itself correctly to null. You can check it like:

if (f)
    // ...