c++ - 使用operator[]时如何区分读/写操作

Question

我需要编写一个带有重载运算符 [] 的类，当使用运算符 [] 读取或写入数据时，它具有不同的行为。举一个我想要实现的实际例子，假设我必须编写一个名为 PhoneBook 的类的实现，它可以按以下方式使用:

PhoneBook phoneBook(999999); // 999999 is the default number which should be
                             // used when calling someone who is not in the phone book

phoneBook["Paul"] = 234657;  // adds Paul's number
phoneBook["John"] = 340156;  // adds John's number

// next line should print Paul's number 234657
cout << "To call Paul dial " << phoneBook["Paul"] << endl;
// next line should print John's number 340156
cout << "To call John dial " << phoneBook["John"] << endl;
// next line should print 999999 because Frank is not in the phone book
cout << "To call Frank dial " << phoneBook["Frank"] << endl;

问题在于使用

phoneBook["Frank"]

我不想在电话簿中添加 Frank 的条目，否则基于 std::map 的解决方案很容易实现。

我没有在网上找到任何标准的方法来实现这个所以经过一番思考，我想到了以下解决方案，其中运算符 [] 返回一个名为 PhoneNumber 的“临时对象”。然后使用 PhoneNumber 来区分读/写操作:

#include <iostream>
#include <string>
#include <map>

using namespace std;

class PhoneBook{
private:
    map<string, int> data_; // stores phone numbers
    int defaultNumber_; // default number returned when no matching name is found

public:
    PhoneBook(int defaultNumber) :
        defaultNumber_(defaultNumber) {}

    // Searches in the phone book for a name. If the name is found it returns
    // the corresponding number. If the name is not found it returns defaultNumber_
    int read(string name){
        map<string, int>::iterator it = data_.find(name);
        if (it==data_.end()){
            return defaultNumber_;
        } else {
            return it->second;
        }
    }

    // Forwarding function to map operator []. It is not really necessary but it is added for clarity
    int& write(string name){
        return data_[name];
    }

    // Forward declaration of the "temporary object" returned by operator []
    // See declaration below
    class PhoneNumber;

    PhoneNumber operator[](string name){
        return PhoneNumber(this, name);
    }

    class PhoneNumber{
        friend class PhoneBook;
    private:
        PhoneBook* const phoneBook_;
        string name_;

        // Constructors are private so that PhoneNumber can be used only by PhoneBook
        // Default constructor should not be used
        PhoneNumber() :
            phoneBook_(NULL) {}

        PhoneNumber(PhoneBook* phoneBook, string name) :
            phoneBook_(phoneBook), name_(name) {}

    public:
        // conversion to int for read operations
        operator int (){
            return phoneBook_->read(name_);
            }

        // assignment operator for write operations
        const int& operator = (const int& val){
            return phoneBook_->write(name_) = val;
        }
    };
};

int main(){
    PhoneBook phoneBook(999999);

    phoneBook["Paul"] = 234657;
    phoneBook["John"] = 340156;

    cout << "To call Paul dial " << phoneBook["Paul"] << endl;
    cout << "To call John dial " << phoneBook["John"] << endl;
    cout << "To call Frank dial " << phoneBook["Frank"] << endl;

    return 0;
}

PhoneBook 类的行为与我希望的一样，程序打印:

To call Paul dial 234657
To call John dial 340156
To call Frank dial 999999

我想问你几个问题:

1. 有没有更好的方法来获得一个与我编写的类行为相似的类？
2. 我使用的技术是否有名称以便我可以搜索更多相关信息？
3. 您是否发现我的解决方案有任何缺点/可能的改进？

在我正在编写的库中，启用我为 PhoneBook::operator[] 获得的行为在类似的情况下真的很重要，我真的很想知道你对我的问题的看法。

谢谢!

Answer 1

您提出的是该问题的标准解决方案。通常是被称为代理模式或代理习语，以及你的助手类返回称为代理。 (因为它是一个嵌套类，只需调用Proxy 通常就足够了。)