【數據結構】27_再論智能指針 （上）

SmartPointer 的設計方案

• 指針生命週期結束時主動釋放堆空間
• 一片堆空間最多隻能由一個指針標識
• 杜絕指針運算和指針比較

新的設計方案

• Poniter 是智能指針的抽象父類（模板）

  • 純虛析構函數 virtual ~Pointer() = 0
  • 重載 operator-> ()
  • 重載 operator* ()

template <typename T>
class Pointer : public Object
{
public:
    Pointer(T *p = NULL);
    T *operator->();
    T &operator*();
    bool isNull();
    T *get();
    
protected:
    T *m_pointer;
};

編程實驗：智能指針的新方案

文件：Pointer.h

#ifndef POINTER_H
#define POINTER_H

#include "Object.h"

namespace DTLib
{

template <typename T>
class Pointer : public Object
{
public:
    Pointer(T *p = nullptr) : m_pointer(p)
    {
    }

    T *operator-> ()
    {
        return m_pointer;
    }

    const T *operator-> () const
    {
        return m_pointer;
    }

    T &operator* ()
    {
        return *m_pointer;
    }

    const T &operator* () const
    {
        return *m_pointer;
    }

    bool isNull() const
    {
        return (m_pointer == nullptr);
    }

    T *get() const
    {
        return m_pointer;
    }

protected:
    T *m_pointer = nullptr;
};

}

#endif // POINTER_H

[重構] 文件： SmartPointer.h

#ifndef SMARTPOINTER_H
#define SMARTPOINTER_H

#include "Pointer.h"

namespace DTLib
{

template<typename T>
class SmartPointer : public Pointer<T>
{
public:
    SmartPointer(T *p = nullptr) : Pointer<T>(p)
    {
    }

    SmartPointer(const SmartPointer &obj) : Pointer<T>(nullptr)
    {
        this->m_pointer = obj.m_pointer;
        const_cast<SmartPointer&>(obj).m_pointer = nullptr;
    }

    SmartPointer &operator= (const SmartPointer &obj)
    {
        if (this != &obj)
        {
            T *pointer = this->m_pointer;

            this->m_pointer = obj.m_pointer;
            const_cast<SmartPointer&>(obj).m_pointer = nullptr;

            delete pointer;
        }

        return *this;
    }

    ~SmartPointer()
    {
        delete this->m_pointer;
    }
};

}

#endif // SMARTPOINTER_H

文件：main.cpp

#include <iostream>
#include "SmartPointer.h"

using namespace std;
using namespace DTLib;

class Test
{
public:
    int i = 10;

    Test()
    {
        cout << "Test()" << endl;
    }

    ~Test()
    {
        cout << "~Test()" << endl;
    }

};

int main()
{
    SmartPointer<Test> sp = new Test();
    SmartPointer<Test> spn;

    cout << sp->i << endl;

    spn = sp;

    cout << spn->i << endl;

    cout << sp.isNull() << endl;
    cout << spn.isNull() << endl;

    return 0;
}

輸出：

Test()
10
10
1
0
~Test()

To be continued...

思考
如何實現 SharedPointer 使得多個智能指針對象能夠指向同一片堆空間，同時支持內存的自動釋放？

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