C++基础之对象组成

    Reusing the implementation

    ◆ Composition: construct new object with existing objects

    ◆ It is relationship of "has-a"

    Composition

    ◆ Objects can be used to build up other objects

    ◆ Ways of inclusion

        — Fully

        — By reference

    ◆ Inclusion by reference allows sharing

    ◆ For example, an Employee has a

        — Name

        — Address

        — Health Plan

        — Salary History

            ★ Collection of Raise objects

        — Supervisor

            ★ Another Employee object!

class Employee { private: int num; Employee* supervisor; };

    Example

#include <iostream> using namespace std; class Person { private: char* name; char* address; public: Person(const char* name, const char* address); ~Person(); void print(); }; class Currency { private: int cents; public: Currency(int, int cents); ~Currency(); void print(); }; class SavingsAccount //创建SavingsAccount的对象时里面会创建两个对象 { public: SavingsAccount(const char* name, const char* address, int cents); ~SavingsAccount(); void print(); private: Person m_saver; Currency m_balance; }; Person::Person(const char* name, const char* address):name((char*)name),address((char*)address) { } Currency::Currency(int a, int cents) : cents(cents) { } SavingsAccount::SavingsAccount(const char* name, const char* address, int cents) : m_saver(name, address), m_balance(0, cents) { } void Person::print() { } void Currency::print() { } void SavingsAccount::print() { m_saver.print(); m_balance.print(); } Person::~Person() { } Currency::~Currency() { } SavingsAccount::~SavingsAccount() { } int main(int argc, const char* argv[]) { SavingsAccount myaccount; return 0; }

    Embedded objects

    ◆ All embedded objects are initialized

        — The default constructor is called if

            ★ you don't supply the arguments, and there is a default constructor( or one can be built)

    ◆ Constructors can have initialization list

        — any number of objects separated by commas

        — is optional

        — Provide arguments to sub-constructors

    ◆ Syntax:

name(args) [':' init-list] '{'

    Question

    ◆ If we wrote the constructor as( assuming we have the set accessors for the subobjects):

SavingAccount::SavingAccount(const char* name, const char* address, int cents) { m_saver.set_name(name); m_saver.set_address(address); m_balance.set_cents(cents); }

    ◆ Default constructors would be called

    Public vs. Private

    ◆ It is common to make embedded objects private:

    — they are part of the underlying implementation

    — the new class only has part of the public interface of the old class

    ◆ Can embed as a public object if you want to have the entire public interface of the subobject available in the new object:

class Person { public: set_name(const char*); }; class Currency { }; class SavingsAccount { public: Person m_saver; Currency m_balance; }; int main(int argc, const char* argv[]) { SavingsAccount account; account.m_saver.set_name("Fred"); //这样，从外部就可以访问Person内的成员函数 return 0; }