【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(0) {}
    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 0;
}

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(0) {}
    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(0), use(new size_t(1)) {}
    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 == 0)
    {
        delete ps;
        delete use;
    }
    ps = rhs.ps;
    i = rhs.i;
    use = rhs.use;
    return *this;
}

HasPtr::~HasPtr()
{
    if (--*use == 0)
    {
        delete ps;
        delete use;
    }
}