智能指针的简单实现

智能指针使用引用计数简单实现：

#include <iostream>
using namespace std;template<class T>
class SmartPtr
{
public:SmartPtr(T *p);	~SmartPtr();SmartPtr(const SmartPtr<T> &orig);                SmartPtr<T>& operator=(const SmartPtr<T> &rhs);   
private:T *ptr;int *use_count;
};template<class T>
SmartPtr<T>::SmartPtr(T *p) : ptr(p)
{try{use_count = new int(1);}catch (...){delete ptr;ptr = nullptr;use_count = nullptr;cout << "Allocate memory for use_count fails." << endl;exit(1);}cout << "Constructor is called!" << endl;
}template<class T>
SmartPtr<T>::~SmartPtr()
{// 只在最后一个对象引用ptr时才释放内存if (--(*use_count) == 0){delete ptr;delete use_count;ptr = nullptr;use_count = nullptr;cout << "Destructor is called!" << endl;}
}template<class T>
SmartPtr<T>::SmartPtr(const SmartPtr<T> &orig)
{ptr = orig.ptr;use_count = orig.use_count;++(*use_count);cout << "Copy constructor is called!" << endl;
}template<class T>
SmartPtr<T>& SmartPtr<T>::operator=(const SmartPtr<T> &rhs)
{// 《C++ primer》：“这个赋值操作符在减少左操作数的使用计数之前使rhs的使用计数加1，// 从而防止自身赋值”而导致的提早释放内存++(*rhs.use_count);// 将左操作数对象的使用计数减1，若该对象的使用计数减至0，则删除该对象if (--(*use_count) == 0){delete ptr;delete use_count;cout << "Left side object is deleted!" << endl;}ptr = rhs.ptr;use_count = rhs.use_count;cout << "Assignment operator overloaded is called!" << endl;return *this;
}int main()
{// Test Constructor and Assignment Operator OverloadedSmartPtr<int> p1(new int(0));p1 = p1;// Test Copy ConstructorSmartPtr<int> p2(p1);// Test Assignment Operator OverloadedSmartPtr<int> p3(new int(1));p3 = p1;return 0;
}