C++ 中[]操作符重载

在实际程序中遇到一操作符问题，记录下来方便以后查询。在写HashTable数据结构中，遇到操作符[]重载问题，具体需求如下:

1.      a[x] = b; //如果a[x]为空，则添加记录

2.      b = a[x]; //如果a[x]为空，则提示出错

对[]操作符重载之后发现根本不能解决问题，查找一些资料后发现该问题为重载后的读写问题，一个重载解决不了问题。上面两点展开如下:

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

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

将a.operator[](x)作为对象A后，问题1就成了对象A的赋值操作符重载问题，问题2就为对象A的类型转换问题。增加一个代理类就可以有效分离两种情形了，具体实现如下：

#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;
}