使用运算符[]时如何区分读/写操作

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，该运算符具有不同的行为。为了给出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，该类可以以以下方式使用：

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;

问题在于当使用时

phoneBook["Frank"]

我不想在Frank的电话簿中添加条目，否则基于std:：map的解决方案将很容易实现。

我在网上没有找到任何标准的方法来实现这一点，所以经过思考，我想出了以下解决方案，其中操作符[]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;
}

类PhoneBook的行为与我想要的一样，程序打印：

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

我想问你一些问题：

1. 有没有更好的方法来获得一个表现得像我编码的类的类
2. 我有没有使用一个名字的技巧，这样我就可以搜索更多关于它的信息
3. 你认为我的解决方案有缺点/可能改进吗

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有什么看法。

谢谢！