37. Smart pointer analysis

Memory leak: dynamically apply for heap space, and do not return it after use. There is no garbage collection mechanism in C++, and the pointer cannot control the life cycle of the pointed heap space.

#include <iostream>
#include <string>
using namespace std;
class Test
{
    int i;
public:
    Test(int i)
    {
        this->i = i;
    }
    int value()
    {
        return i;
    }
    ~Test()
    {
    }
};
int main()
{
    for(int i=0; i<5; i++)
    {
        Test* p = new Test(i);                               //没有归还空间
        
        cout << p->value() << endl;            
    }   
    return 0;

}

A special pointer is required, and the heap space is actively released at the end of the life cycle,

A piece of heap space can only be identified by at most one pointer (only copy constructors and overloaded assignment operators are required)

Avoid pointer arithmetic and pointer comparisons.

Solution:

Overloading pointer trait operators (-> and *)

It can only be overloaded through the member function of the class, the overloaded function cannot use parameters, and only one overloaded function can be defined.

#include <iostream>
#include <string>
using namespace std;
class Test
{
    int i;
public:
    Test(int i)
    {
        cout << "Test(int i)" << endl;
        this->i = i;
    }
    int value()
    {
        return i;
    }
    ~Test()
    {
        cout << "~Test()" << endl;
    }
};
  //智能指针
class Pointer
{
    Test* mp;
public:
    Pointer( Test* p = NULL )
    {
        mp = p;
    }
    Pointer(const Pointer& obj)
    {
        mp = obj.mp;
        const_cast<Pointer&>(obj).mp = NULL;        //去掉只读属性
    }
    Pointer& operator = (const Pointer& obj)
    {
        if( this != &obj )
        {
            delete mp;
            mp = obj.mp;
            const_cast<Pointer&>(obj).mp = NULL;
        }        
        return *this;
    }
    Test* operator -> ()
    {
        return mp;
    }
    Test& operator * ()
    {
        return *mp;
    }
    bool isNull()
    {
        return (mp == NULL);
    }
    ~Pointer()
    {
        delete mp;
    }
};

int main()
{
    Pointer p1 = new Test(0);                         //用pointer类对象代替指针
    
    cout << p1->value() << endl;
    
    Pointer p2 = p1;
    
    cout << p1.isNull() << endl;
    
    cout << p2->value() << endl;
    
    return 0;

}

Smart pointers: simulate the behavior of pointers by replacing pointers with an object.

Military rules for the use of smart pointers: can only be used to point to objects or variables in the heap space,

Pointer feature operators (-> and *) can be overloaded. Overloaded pointer feature operators can use objects instead of pointers to avoid memory problems to the greatest extent.