C++ 中[]操作符重載

在實際程式中遇到一操作符問題，記錄下來友善以後查詢。在寫HashTable資料結構中，遇到操作符[]重載問題，具體需求如下:

1.      a[x] = b; //如果a[x]為空，則添加記錄

2.      b = a[x]; //如果a[x]為空，則提示出錯

對[]操作符重載之後發現根本不能解決問題，查找一些資料後發現該問題為重載後的讀寫問題，一個重載解決不了問題。上面兩點展開如下:

1.      a.operator[](x).opeator=(b) // A.operator=(b)

2.      b.operator=(a.operator[](x)) //b.operator=(A)

将a.operator[](x)作為對象A後，問題1就成了對象A的指派操作符重載問題，問題2就為對象A的類型轉換問題。增加一個代理類就可以有效分離兩種情形了，具體實作如下：

#include <iostream>
#include <vector>

using namespace std;

class A {
	
private:
	vector<int> v;
	int defnum;
public:
	A(unsigned int number):defnum(-1) {
		for(unsigned int i = 0; i < 10; ++i) {
			v.push_back(number);
		}
	}

	
	const int& operator[](const int &u) const {
		cout << "operator[](const)  const -- reading, u:"<< u << ",value:" << v[u] << endl;
		if(v[u] == 0) return defnum;
		return v[u];
	}

	int & operator[](const int &u) {
		cout << "const operator[](const)--writing,u:"<< u << ",value:" << v[u] << endl;
		return v[u];
	}
	
};

class B {
private:
	vector<int> v;
	int defnum;

public:
	B(unsigned int number):defnum(-1) {
		for(unsigned int i = 0; i < 10; ++i) {
			v.push_back(number);
		}
	}
	
private:
	int read(int idx)  {
		if(v[idx] == 0) return defnum;
		return v[idx];
	}
	
	const int&  write(int idx, int value) {
		v[idx] = value;
		return v[idx];
	}
	
	class Proxy {
		friend class B;
	private:
		B * const pb_;
		int v_;
		
		Proxy(): pb_(NULL),v_(0) {}
		Proxy(B *pb, int idx):pb_(pb),v_(idx) {}
		
	public:
		operator int() {
			cout << "reading in here" << endl;
			return pb_->read(v_);
		}

		const int& operator = (const int &val) {
			cout << "writing in here" << endl; 
			return pb_->write(v_, val);
		}
	};
public:
		
	Proxy operator[](const int &idx) {
		return Proxy(this,idx);
	}

};

int main(int argc, char *argv[]) {
	
	A  a(0);
	B b(0);
	int value;

	a[1] = 3; // writing
	value = a[2]; //reading
	value = 4;
	cout << "a[1]:" << a[1] << ",value:"<< value << ",a[2]:" << a[2]<<endl;


	b[1] = 3; // writing
	value = b[2]; //reading
	value = 4;
	cout << "b[1]:" << b[1] << ",value:"<< value << ",b[2]:" << b[2]<<endl;

	return 0;
}