【C++ Primer 第13章】2. 拷贝控制和资源管理

拷贝控制和资源管理

• 类的行为像一个值。意味着它应该有自己的状态，当我们拷贝一个像值得对象时，副本和原对象是完全独立的，改变副本不会对原对象有任何影响。

• 行为像指针的类则共享状态。当我们拷贝一个这种类的对象时，副本和原对象使用相同的底层数据，改变副本也会改变原对象。

13.2节练习

 #include<iostream>
 #include<string>
 using namespace std;

 class HasPtr {
 public:
     HasPtr(const string &s = string()): ps(new string(s)), i() {}
     HasPtr(const HasPtr &rhs): ps(new string(*rhs.ps)), i(rhs.i) {}

     HasPtr &operator=(const HasPtr &rhs);
     HasPtr &operator=(const string &rhs);
     string &operator*();

     ~HasPtr() { delete ps; }
 private:
     string *ps;
     int i;
 };

 HasPtr &HasPtr::operator=(const HasPtr &rhs)
 {
     auto newp = new string(*rhs.ps);
     delete ps;
     ps = newp;
     i = rhs.i;
     return *this;
 }

 HasPtr &HasPtr::operator=(const string &rhs)
 {
     *ps = rhs;
     return *this;
 }

 string& HasPtr::operator*()
 {
     return *ps;
 }

 int main()
 {
     HasPtr h("hi mom!");
     HasPtr h2(h);
     HasPtr h3(h);
     h2 = "hi dad!";
     h3 = "hi, son!";
     cout << "h: " << *h << endl;
     cout << "h2: " << *h2 << endl;
     cout << "h3: " << *h3 << endl;
     return ;
 }

运行结果：

行为像值的类

 #include<iostream>
 #include<string>
 using namespace std;

 class HasPtr {
 public:
     HasPtr(const string &s = string()): ps(new string(s)), i() {}
     HasPtr(const HasPtr &rhs): ps(new string(*rhs.ps)), i(rhs.i) {}

     HasPtr& operator=(const HasPtr &rhs);
     ~HasPtr() { delete ps; }
 private:
     string *ps;
     int i;
 };

 HasPtr& HasPtr::operator=(const HasPtr &rhs)
 {
     auto newp = new string(*rhs.ps);
     delete ps;
     ps = newp;
     return *this;
 }

定义行为像指针的类

 #include<iostream>
 #include<string>
 using namespace std;

 class HasPtr {
 public:
     HasPtr(const string &s = string()): ps(new string(s)), i(), use(new size_t()) {}
     HasPtr(const HasPtr &rhs): ps(rhs.ps), i(rhs.i), use(rhs.use) { ++*use; }

     HasPtr &operator=(const HasPtr &rhs);
     ~HasPtr();

 private:
     string * ps;
     int i;
     size_t *use;  //用来记录有多少个对象共享*ps的成员
 };

 HasPtr &HasPtr::operator=(const HasPtr &rhs)
 {
     ++*rhs.use;
     if (--*use == )
     {
         delete ps;
         delete use;
     }
     ps = rhs.ps;
     i = rhs.i;
     use = rhs.use;
     return *this;
 }

 HasPtr::~HasPtr()
 {
     if (--*use == )
     {
         delete ps;
         delete use;
     }
 }