Consider the following class:

#include "OtherData.h"class MyClass {  OtherData m_data;public:  bool func(OtherData data);};

It seems as though this class hides its implementation details efficiently, but is actually doesn’t! The user knows about the OtherData type and its requirements in its header file. To overcome this situation, you can do this:

class OtherData; // forward refernceclass MyClass {  OtherData *m_pData;public:  bool func(const OtherData &data);};

But what if you’ve got more than one field? Suppose you have a forward reference per data type. Now, you can define another extra class and store all the data there:

class DataHolder;class MyClass {  DataHolder *m_pDataHolder;public:  void func1(const OtherData1 &data);  bool func2(const OtherData2 &data);};

Now all the data are defined in DataHolder, which is then hidden efficiently.