Use of PIMPL Design Pattern

Introduction

PIMPL (Private Implementation) is a simple way to hide a part of a class implementation to other classes of the same project.

It makes it possible to avoid other classes to know internal data structures and other information of the class. It also simplifies some #include preprocessor instructions.

Example WITHOUT PIMPL

We have a CFoo class which is defined in two files foo.h and foo.cpp:

Our main instantiates this class to run one of its functions.

File foo.h

class CFoo
{
public:
    CFoo();
    ~CFoo();
    bool ProcessFile(const CString & csFile);
private:
    CFooInternalData    m_data;
    CHeader             m_header;
} 

File foo.cpp

#include "FooInternalData.h"
#include "Header.h"
#include "foo.h"
CFoo::CFoo()
{
}

CFoo::~CFoo()
{
}

bool CFoo::ProcessFile(const CString & csFile)
{
    //do something
    return true;
}

Main File

#include "FooInternalData.h"
#include "Header.h"
#include "foo.h"

int main()
{
    CFoo foo;
    foo.ProcessFile("c:\\data.bin");
    return 0;
} 

The problem with this simple way of coding is that in your main file, you must include the foo.h to use it, but at the same time you must also include all needed files to allow the compiler to work correctly. In fact, the main does not need to include FooInternalData.h and Header.h (which are CFoo internal structures) except for compilation.... So with very big classes, you might do some huge includes and in this case, you can have some compiler or linker errors because files are already included elsewhere.

The Same Example with PIMPL

File foo.h

//here just declare the class PIMPL to compile. 
//As I use this class with a pointer, I can use this declaration 
class CFoo_pimpl; 

class CFoo
{
public:
    CFoo();
    ~CFoo();
    bool ProcessFile(const CString & csFile);
private:
    std::auto_ptr<CFoo_pimpl>    m_pImpl;
}  

File foo.cpp

#include "FooInternalData.h"
#include "Header.h"
#include "foo.h"

//here defines PIMPl class, because it is use only in this file
class CFoo_pimpl()
{
public:
    CFoo_pimpl()
    {
    }

    ~CFoo_pimpl()
    {
    }  
    bool ProcessFile(const CString & csFile)
    {
        //do something
        return true;
    }
};

CFoo::CFoo()
:m_pImpl(new CFoo_pimpl())
{
}

CFoo::~CFoo()
{
    //do not have to delete, std::auto_ptr is very nice 
}

bool CFoo::ProcessFile(const CString & csFile)
{
    //just call your PIMPL function ;-)
    return m_pImpl->ProcessFile(csFile);
}

Main File

#include "foo.h"
int main() 
{
    CFoo foo;
    foo.ProcessFile("c:\\data.bin");
    return 0; 
} 

The result is obvious: simplicity of use!! The main does not need more includes for internal structures of CFoo class.

Thus it is an excellent optimization to minimize linker and compiler errors.

Conclusion

It is a very simple and nice way for good coding!!! If you want to use classes in other projects, it does not introduce including difficulties.

Unfortunately, you must add some more code to type.

History

• 6^th February, 2007: Initial post