Question

I’d need to write a class with an overloaded operator [] which has different behavior when the operator [] is used to read or write data.
To give a practical example of what I want to achieve, let’s say I have to write the implementation of a class named PhoneBook which can be used in the following way:

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;

The problem is in the fact that when using

phoneBook["Frank"]

I don’t want to add an entry in the phone book for Frank, otherwise a solution based on std::map would be easy to implement.

I did not find on the web any standard way to achieve this so
after some thinking I came up with the following solution in which the operator [] returns a “temporary object” named PhoneNumber. PhoneNumber is then used to distinguish between read/write operations:

#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;
}

The class PhoneBook behaves like I would like and the program prints:

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

I would like to ask you some questions:

1. Is there any better way to obtain a class behaving like the class I coded?
2. Has the technique I’m using a name so that I can search more info about it?
3. Do you see any drawback/possible improvement in my solution?

In the library I’m writing, enabling the behavior I obtained for PhoneBook::operator[]
in a similar situation is really important and I would really like to know what you think about my problem.

Thanks!

Answer 1

What you propose is the standard solution to this problem. It’s usually
known as the proxy pattern or proxy idiom, and the helper class that you
return is called a proxy. (Since it is a nested class, simply calling
it Proxy is generally sufficient.)