C++ 智能指针

A smart pointer is a class object that acts like a pointer but has addiction features. --《C++ Primer》第六版

一、使用普通指针的问题

1) 每次调用 remodel，动态分配内存，最后没有释放该资源，将导致memory leak.

void remodel(std::string& str)
{
    std::string* ps = new std::string(str);//动态分配内存
    ...
    str = ps;
    return;
}

2) 即使最后释放资源，但是在中间抛出异常，则delete 语句无法执行，仍导致memory leak.

void remodel(std::string& str)
{
    std::string* ps = new std::string(str);//动态分配内存
    ...
    if(weird_thing())//抛出异常
       throw_exception();
    str = *ps;
    delete ps;//释放动态分配的内存
    return;
}

 二、智能指针使用实例

主要有三种智能指针，auto_ptr/unique_ptr/shared_ptr。定义一个指向类A的智能指针pA：auto_ptr<A> pA(new A)。  

#include <iostream>
#include <string>
#include <memory>
using namespace std;
class Report
{
public:
    Report(const string s):str(s){
        cout<<"Object created!\n";
    }
    ~Report(){
        cout<<"Object deleted!\n";
    }
    void comment() const{
        cout<<str<<"\n";
    }
private:
    string str;
};

int main()
{
    {//auto_ptr
        auto_ptr<Report> ps (new Report("using auto_ptr"));
        ps->comment(); // use -> to invoke a member function
    }
    cout<<"\n";
    {//shared_ptr
        shared_ptr<Report> ps (new Report("using shared_ptr"));
        ps->comment();
    }
    cout<<"\n";
    {//unique_ptr
        unique_ptr<Report> ps (new Report("using unique_ptr"));
        ps->comment();
    }
    return 0;
}

结果：当指针生命周期结束时，会自动调用所指向对象的析构函数

 三、使用智能指针的注意

1) 普通指针和智能指针间需要通过显示转换

智能指针有显示构造函数，它的参数是指针。普通指针需要显示转换成智能指针：Each of these classes has an explicit constructor taking a pointer as an argument. Thus, there is no automatic type cast from a pointer to a smart pointer object. 

shared_ptr<double> pd;
double *p_reg = new double;
pd = p_reg; // not allowed (implicit conversion)
pd = shared_ptr<double>(p_reg); // allowed (explicit conversion
shared_ptr<double> pshared = p_reg; // not allowed (implicit conversion)
shared_ptr<double> pshared(p_reg); // allowed (explicit conversion)

 2) 智能指针不能非动态分配的对象作为构造函数的参数

因为它最后会用delete 操作来释放该资源。When pvac expires, the program would apply the delete operator to non-heap memory, which is wrong.

string vacation("I wandered lonely as a cloud.");
shared_ptr<string> pvac(&vacation); // NO!

四、为什么少使用auto_ptr？

1) auto_ptr and unique_ptr 策略

多个智能指针不能同时指向同一个对象。假设 A 指向 C, 当使用 B=A 赋值后，原来的智能指针A就失去了对C的所有权，即不能通过A来访问对象C。

Institute the concept of ownership, with only one smart pointer allowed to own a particular object. Only if the smart pointer owns the object will its destructor delete the object.Then have assignment transfer ownership.This is the strategy used for auto_ptr and for unique_ptr, although unique_ptr is somewhat more restrictive.

2) shared_ptr 策略

引用计数：每次赋值会使计数器递增；指针过期会使计数器递减；当最后一个指针过期（计数器为0）将调用 delete。

Create an even smarter pointer that keeps track of how many smart pointers refer to a particular object.This is called reference counting.Assignment, for example, would increase the count by one, and the expiration of a pointer would decrease the count by one. Only when the final pointer expires would delete be invoked.

3）auto_ptr 实例

多个auto_ptr指向同一个对象，会使原来的指针失效，非法访问

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

int main()
{
    //files：一个包含５个string 对象的数组
    auto_ptr<string> files[5] = 
    {
        auto_ptr<string> (new string("Fowl balls")),
        auto_ptr<string> (new string("Duck Walks")),
        auto_ptr<string> (new string("Chicken Runs")),
        auto_ptr<string> (new string("Turkey Errors")),
        auto_ptr<string> (new string("Goose Eggs"))
    };

    auto_ptr<string> pwin;
    pwin = files[2]; //files[2] 失去对象的ownership
    cout << "The nominees for best avian baseball film are\n";
    for (int i = 0; i < 5; i++)
        cout<<*files[i]<<endl;
    cout<<"The winner is "<<*pwin<<"!\n";
    return 0;
}

4) unique_ptr

当多个unique_ptr指向同一个对象，会产生编译错误

unique_ptr< string> pu1(new string "Hi ho!");
unique_ptr< string> pu2;
pu2 = pu1; //#1 not allowed
unique_ptr<string> pu3;
pu3 = unique_ptr<string>(new string "Yo!"); //#2 allowed

unique_ptr可以使用new[](delete[])。The auto_ptr template uses delete, not delete [], so it can only be used with new, not with new []. But unique_ptr has a new[], delete[] version: std::unique_ptr< double[]>pda(new double(5)); // will use delete []

 五、如何选择智能指针

1) shared_ptr

程序里需要用多个指针指向同一个对象。比如：拥有一个数组指针，且需要标识出一些特别的元素（最大/最小)；有两个对象都需要指向第三个类的对象；使用STL容器，容器的对象为指针。

2) unique_ptr

程序里不需要多个指针指向同一个对象。比如：把unique_ptr 作为函数的返回值，它指向一块动态分配的内存。

unique_ptr<int> make_int(int n){
    return unique_ptr<int>(new int(n));
}