将文件间的编译依赖关系（Compilation dependency）降至最低

//如果如下头文件中有任何一个改变，或者这些头文件所依赖的其他头文件有任何改变，那么每一个#include "Person.h"就得重新编译，任何使用Person Class的文件也必须重新编译。

==handle class
#include
#include “date.h”
#include “address.h”
[Person.h更改前]
Person {
public:
Person(const std：：string& name, const Date& birthday, const Address& addr);
std::string name() const;
std::string birthday() const;
std::string address() const;
private:
std::string theName;
Date theBirthDate;
Address theAddress;
};

[Person.h更改后] 这样更改后#include "Person.h"的文件，不会因为PersonImpl, Date, Address的改变而重新编译,因为客户include "Person.h"文件中没有关于PersonImpl, Date, Address它们的定义式include “PersonImpl.h”，而是用了类的声明式class PersonImpl;
#include
class PersonImpl; //类的声明式
class Date;
class Address;

class Person {
public:
Person(const std::string& name, const Date& birthday, const Address& addr);
std::string name() const;
std::string birthdayDate() const;
std::string address() const;
private:
std::tr1:shared_ptr pImpl;
};

[Person.cpp]
#include “Person.h”
#include “PersonImpl.h”

Person::Person(const std：：string& name, const Date& birthday, const Address& addr)

pImpl(new PersonImpl(name, birthday, addr))
()
std::string Person::name() const
{
return pImpl->name();
}

interface class=
class Person {
public:
virtual ~Person();
virtual std::string name() const=0;
virtual std::string birthDate() const=0;
virtual std::string address() const=0;
static std::tr1::shared_ptr create(const std::string& name, const Date& birthday, const Address& addr);
};

客户会这样使用它们：
std::string name;
Date dateOfBirth;
Address address;
std::tr1::shared_ptr pp(Person::create(name, address, address));
std::out << pp->name()
<< “was born on "
<< pp->birthDate()
<< " and now lives at”
<< pp->adress();

class RealPerson : public Person {
public:
RealPerson(const std::string& name, const Date& birthday, const Address& addr)
: theName(name), theBirthDate(birthday), theAddress(addr)
()
virtual ~RealPerson() {}
std::string name() const;
std::string birthdayDate() const;
std::string address() const;
private:
std::string theName;
Date theBirthDate;
Address theAddress;
};

std::tr1::shared_ptr Person::create(const std::string& name, const Date& birthday, const Address& addr);
{
return std::tr1::shared_ptr(new RealPerson(name, birthday, addr));
}

支持“编译依存性最小化”的一般构想是：相依于声明式，不要相依于定义式。
程序库头文件应该以“完全且仅有声明式”的形式存在。