auto_ptr与shared_ptr

注: 从c++11开始, auto_ptr已经被标记为弃用, 常见的替代品为shared_ptr
shared_ptr的不同之处在于引用计数, 在复制(或赋值)时不会像auto_ptr那样直接转移所有权

auto_ptr

• auto_ptr实际也是一种类, 拥有自己的析构函数, 生命周期结束时能自动释放资源
• 正因为能自动释放资源, 特别适合在单个函数内代替new/delete的调用, 不用自己调用delete也不用担心意外退出
• auto_ptr对资源的拥有者只能有一个, 当两个auto_ptr进行等于号(赋值)操作时, 等于号后面的auto_ptr将失去资源的所有权
• 如果只是想显示auto_ptr对应资源的信息, 而不是更改的话, 要以const auto_ptr&(const + 引用)的方式来传递给其它函数

赋值与安全传递

#include <iostream>
#include <memory>

using namespace std;
template<class T>
ostream& operator<< (ostream& os,const auto_ptr<T>& p){
    if(p.get() == NULL)
        os<<"NULL";
    else
        os<<*p;
    return os;
}
int main(int  argc,char *argv[]){
    auto_ptr<int> p(new int(42));
    auto_ptr<int> q;

    cout<<"after initialization"<<endl;
    cout<<"p:"<<p<<endl;
    cout<<"q:"<<q<<endl;
    
    q=p;
    cout<<"after assign"<<endl;
    cout<<"p:"<<p<<endl;
    cout<<"q:"<<q<<endl;

    *q += 13;
    p=q;
    cout<<"another assign"<<endl;
    cout<<"p:"<<p<<endl;
    cout<<"q:"<<q<<endl;
    return 0;
}

非安全传递
以下与上面的例子差不多也只是显示auto_ptr所指对象的值, 以非const 引用的方向传递, 结果在函数调用中就把资源给释放了

#include <iostream>
#include <memory>

using namespace std;
template<class T>
ostream& operator<< (ostream& os,auto_ptr<T> p){
    if(p.get() == NULL)
        os<<"NULL";
    else
        os<<*p;
    return os;
}
int main(int  argc,char *argv[]){
    auto_ptr<int> p(new int(42));
    cout<<p<<endl;

    if(p.get() == NULL)
        cout<<"after call function, p = NULL"<<endl;
    return 0;
}

shared_ptr

#include <iostream>
#include <tr1/memory>

using namespace std;

class base{
    public:
        base(){cout<<"base init"<<endl;}
        virtual ~base(){cout<<"base over"<<endl;}
        virtual void show(){cout<<"from base"<<endl;}
};

class derived: public base{
    public:
        derived(){cout<<"derived init"<<endl;}
        ~derived(){cout<<"derived over"<<endl;}
        void show(){cout<<"from derived"<<endl;}
};

int main () {
    tr1::shared_ptr<derived> ptr(new derived());
    tr1::shared_ptr<derived> ptr2(ptr);
    cout<<ptr.get()<<endl;
    cout<<ptr2.get()<<endl;

    class derived *dptr=ptr.get();
    dptr->show();

    return 0;
}