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.
Note that wxWidgets follows the STL philosophy: when a comparison operator can not be implemented efficiently (like for e.g. wxImage'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 wxObject derived classes, you should not use the == and != operators but rather the wxObject::IsSameAs() function.
All the objects implement a function IsOk() to test if they are referencing valid data; when the objects are in uninitialized state, you can only use the IsOk() getter; trying to call any other getter, e.g. wxBrush::GetStyle() on the wxNullBrush object, will result in an assert failure in debug builds.
First, derive a new class from wxObjectRefData and put there the memory-consuming data.
Then derive a new class from wxObject and implement there the public interface which will be seen by the user of your class. You'll probably want to add a function to your class which does the cast from wxObjectRefData to your class-specific shared data. For example:
MyClassRefData* GetData() const { return wx_static_cast(MyClassRefData*, m_refData); }
In fact, any time you need to read the data from your wxObject-derived class, you will need to call this function.
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