Reference Counting Overview

Why You Shouldn’t Care About It

Many of the C++ objects wrapped by wxPython use a technique known as reference counting, also known as copy on write (COW). This means that when an object is assigned to another, no copying really takes place. Only the reference count on the shared object data is incremented and both objects share the same data (a very fast operation).

But as soon as one of the two (or more) objects is modified, the data has to be copied because the changes to one of the objects shouldn’t be seen in the others. As data copying only happens when the object is written to, this is known as COW.

What is important to understand is that all this happens absolutely transparently to the class users and that whether an object is shared or not is not seen from the outside of the class - in any case, the result of any operation on it is the same.

Object Comparison

The == and != operators of the reference counted classes always do a deep comparison. This means that the equality operator will return true if two objects are identical and not only if they share the same data.

Note that wxPython follows the STL philosophy: when a comparison operator cannot be implemented efficiently (like for e.g. wx.Image‘s == operator which would need to compare the entire image’s data, pixel-by-pixel), it’s not implemented at all. That’s why not all reference counted classes provide comparison operators.

Also note that if you only need to do a shallow comparison between two wx.Object derived classes, you should not use the == and != operators but rather the wx.Object.IsSameAs() function.

Object Destruction

When a COW object destructor is called, it may not delete the data: if it’s shared, the destructor will just decrement the shared data’s reference count without destroying it. Only when the destructor of the last object owning the data is called, the data is really destroyed. Just like all other COW-things, this happens transparently to the class users so that you shouldn’t care about it.

List of Reference Counted Classes

The following classes in wxPython have efficient (i.e. fast) assignment operators and copy constructors since they are reference-counted:

• wx.AcceleratorTable
• wx.adv.Animation
• wx.Bitmap
• wx.Brush
• wx.Cursor
• wx.Font
• wx.GraphicsBrush
• wx.GraphicsContext
• wx.GraphicsFont
• wx.GraphicsMatrix
• wx.GraphicsPath
• wx.GraphicsPen
• wx.Icon
• wx.Image
• wx.msw.Metafile
• wx.Palette

Note that the list above reports the objects which are reference counted in all ports of wxPython; some ports may use this technique also for other classes.