问题与C ++中的代理模式(Issue with Proxy Pattern in c++)

 我被告知在我的程序中使用代理模式，这对我来说不是很清楚。 
 
 我有一些Proxy &operator*() ，我不知道应该返回什么来获取文件中当前索引的值。 
 
 我之前有这个并且它有效： 
 
int &operator*()
{
    return ptr->getValue(index);
}
 
 但我被告知要将其更改为Proxy 。 
 
 如果我使用Proxy &operator*()进行编译，那么我就会收到 
 
 
 
 错误：从'int'类型的表达式'IntFile :: Proxy＆'类型的引用的无效初始化 
 
 
 完整代码： 
 
#include <iostream>
#include <cstdio>
#include <vector>

using namespace std;

class IntFile
{
public:
    int value;
    FILE* file;

    IntFile()
    {
        file = fopen("text.txt", "r+b");
    }

    ~IntFile()
    {
        fclose(file);
    }

    virtual int& getValue(int index)
    {
        fseek(file, 4*index, SEEK_SET);
        fread(&value, 4, 1, file);
        return value;
    }

    virtual int setValue(int value)
    {
        fseek(file, 0, SEEK_CUR);
        fwrite(&value, 4, sizeof(value),file);
        return 0;
    }

    class Proxy
    {
    private:
        IntFile *ptr;
        long index;
    public:
        Proxy(IntFile* ptr3)
        {
            ptr = ptr3;
        }
        Proxy& operator=(int value)
        {
            ptr->setValue(value);
        }
        operator int() const
        {
            return ptr->getValue(index);
        }
    };

    friend struct iterator;
    struct iterator
    {
        int index;
        int value2;
        IntFile* ptr;

        iterator(IntFile* ptr2, int idx, FILE* ptrfile)
        {
            ptr = ptr2;
            index = idx;
            fseek(ptrfile, 4*index, SEEK_SET);
        }

        bool operator==(const iterator&other) const
        {
            return index == other.index;
        }

        bool operator!=(const iterator&other) const
        {
            return index!=other.index;
        }

        Proxy &operator*()
        {
            // How to do that?
        }

        int &operator=(int value)
        {
            this->value2 = value;
        }

        iterator&operator++()
        {
            this->index = index+1;
        }

        iterator&operator--()
        {
            this->index = index -1;
        }
    };

    iterator begin()
    {
        return iterator(this, 0, file);
    }

    iterator end(int number)
    {
        return iterator(this, number, file);
    }

    iterator rbegin(int number)
    {
        return iterator(this, number-1, file);
    }

    iterator rend()
    {
        return iterator(this, -1, file);
    }
};

int main()
{
    IntFile myfile;

    int number;
    cout << "Enter number of elements: " << endl;
    cin >> number;

    vector <int> myVector;

    cout << "Enter your numbers: ";
    for ( int i = 0; i < number; i++)
    {
        cin >> myfile.value;
        myVector.push_back(myfile.value);
    }

    fwrite(&myVector[0], sizeof(vector<int>::value_type), myVector.size(),myfile.file);

    cout << endl << "FORWARD 1 by 1: " << endl;
    for (IntFile::iterator i = myfile.begin(); i != myfile.end(number); ++i)
    {
        cout << *i << " ";
    }

    cout << endl << "BACKWARD 1 by 1: " << endl;
    for (IntFile::iterator i = myfile.rbegin(number); i != myfile.rend(); --i)
    {
        cout << *i << " ";
    }

    cout << endl;
    return 0;
}

I was told to use proxy pattern in my program which is not that clear to me. 
 
I have some issues with Proxy &operator*() , I don't know what should I return there to get a value of current index in file.
 
I had this before and it worked:
 
int &operator*()
{
    return ptr->getValue(index);
}
 
But I was told to change it to Proxy. 
 
If I compile with Proxy &operator*() then I'm getting 
 
 
 
error: invalid initialization of reference of type 'IntFile::Proxy&' from expression of type 'int'|
 
 
Full code:
 
#include <iostream>
#include <cstdio>
#include <vector>

using namespace std;

class IntFile
{
public:
    int value;
    FILE* file;

    IntFile()
    {
        file = fopen("text.txt", "r+b");
    }

    ~IntFile()
    {
        fclose(file);
    }

    virtual int& getValue(int index)
    {
        fseek(file, 4*index, SEEK_SET);
        fread(&value, 4, 1, file);
        return value;
    }

    virtual int setValue(int value)
    {
        fseek(file, 0, SEEK_CUR);
        fwrite(&value, 4, sizeof(value),file);
        return 0;
    }

    class Proxy
    {
    private:
        IntFile *ptr;
        long index;
    public:
        Proxy(IntFile* ptr3)
        {
            ptr = ptr3;
        }
        Proxy& operator=(int value)
        {
            ptr->setValue(value);
        }
        operator int() const
        {
            return ptr->getValue(index);
        }
    };

    friend struct iterator;
    struct iterator
    {
        int index;
        int value2;
        IntFile* ptr;

        iterator(IntFile* ptr2, int idx, FILE* ptrfile)
        {
            ptr = ptr2;
            index = idx;
            fseek(ptrfile, 4*index, SEEK_SET);
        }

        bool operator==(const iterator&other) const
        {
            return index == other.index;
        }

        bool operator!=(const iterator&other) const
        {
            return index!=other.index;
        }

        Proxy &operator*()
        {
            // How to do that?
        }

        int &operator=(int value)
        {
            this->value2 = value;
        }

        iterator&operator++()
        {
            this->index = index+1;
        }

        iterator&operator--()
        {
            this->index = index -1;
        }
    };

    iterator begin()
    {
        return iterator(this, 0, file);
    }

    iterator end(int number)
    {
        return iterator(this, number, file);
    }

    iterator rbegin(int number)
    {
        return iterator(this, number-1, file);
    }

    iterator rend()
    {
        return iterator(this, -1, file);
    }
};

int main()
{
    IntFile myfile;

    int number;
    cout << "Enter number of elements: " << endl;
    cin >> number;

    vector <int> myVector;

    cout << "Enter your numbers: ";
    for ( int i = 0; i < number; i++)
    {
        cin >> myfile.value;
        myVector.push_back(myfile.value);
    }

    fwrite(&myVector[0], sizeof(vector<int>::value_type), myVector.size(),myfile.file);

    cout << endl << "FORWARD 1 by 1: " << endl;
    for (IntFile::iterator i = myfile.begin(); i != myfile.end(number); ++i)
    {
        cout << *i << " ";
    }

    cout << endl << "BACKWARD 1 by 1: " << endl;
    for (IntFile::iterator i = myfile.rbegin(number); i != myfile.rend(); --i)
    {
        cout << *i << " ";
    }

    cout << endl;
    return 0;
}

原文：https://stackoverflow.com/questions/47049524