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Pimpl is an abbreviation of 'pointer to
implementation'.
The idea of the Pimpl idiom is to give a class (for
example 'Lizard') an opaque (smart) pointer
to the actual implementation (for example 'LizardImpl'). The
opaque (smart) pointer enables it to do a
forward declaration of the implementation
class only, without the compiler needing to know
the actual type. This shortens build times [1]. All the
public member functions of the Pimpl
class will (often) call the member functions of the
implementation class with the same name.
The advantages of using the Pimpl idiom are:
The disadvantage of using the Pimpl idiom is the cost of
an extra level of indirection, so Pimpl judiciously [1].
Most lizards remain having the same gender for all their live. Therefore, it is a
good idea to make a lizard's gender a const member variable. Problem is, that
this makes a lizard class uncopyable. In this example I solve this by making a
Lizard contain an opaque pointer to LizardImpl, where a LizardImpl does have a
constant gender. Because I want to be able to do a shallow
copy on Lizards, I use a boost::shared_ptr.
Also note that the code is very similar to a Strategy
design pattern.
lizard.h
//---------------------------------------------------------------------------
// UnitLizard.h
//---------------------------------------------------------------------------
#ifndef UnitLizardH
#define UnitLizardH
//---------------------------------------------------------------------------
#include <boost/shared_ptr.hpp>
//---------------------------------------------------------------------------
enum Gender { male, female };
struct Lizard
{
Lizard(const Gender gender);
const Gender GetGender() const;
private:
struct LizardImpl;
boost::shared_ptr<LizardImpl> mPimpl;
};
#endif
|
lizard.cpp
//---------------------------------------------------------------------------
// UnitLizard.cpp
//---------------------------------------------------------------------------
#include "UnitLizard.h"
//---------------------------------------------------------------------------
struct Lizard::LizardImpl
{
LizardImpl(const Gender gender);
const Gender mGender;
};
//---------------------------------------------------------------------------
Lizard::Lizard(const Gender gender)
: mPimpl(boost::shared_ptr<LizardImpl>(new LizardImpl(gender) ) )
{
}
//---------------------------------------------------------------------------
const Gender Lizard::GetGender() const
{
return mPimpl->mGender;
}
//---------------------------------------------------------------------------
// The actual Lizard implementation
//---------------------------------------------------------------------------
Lizard::LizardImpl::LizardImpl(const Gender gender) : mGender(gender)
{
}
//---------------------------------------------------------------------------
|
- Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Item 43: 'Pimpl judiciously'.
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