C++ ：学习Proxy Class之笔记2

依然是对more effective c＋＋条款30－Proxy Class的学习笔记。

#include
using namespace std;

template
class Array
{
public:
class Proxy
{
public:
Proxy(Array& array, int index)
: theArray(array), itemIndex(index)
{}

Proxy& operator = (const T& rhs)
{
theArray.array[itemIndex] = rhs;
return *this;
}
operator T() const
{
return theArray.array[itemIndex];
}

friend ostream& operator<< (ostream& os, const Proxy& rhs)
{
os << rhs.theArray[rhs.itemIndex];

return os;
}

private:
Array& theArray;
int itemIndex;
};

const Proxy operator[](int index) const;
Proxy operator[](int index);

friend class Proxy;
Array(int dim);
~Array();

private:
T* array;
};

template
Array::Array(int dim)
{
array = new T[dim];
}

template
Array::~Array()
{
delete [] array;
}

template
Array::Proxy Array::operator[](int index)
{
return Proxy(*this, index);
}

template
const Array::Proxy Array::operator [] (int index) const
{
return Proxy(const_cast(*this), index);
}

class Rational
{
public:
Rational(int numerator = 0, int denominator = 1);
int numerator() const;
int denominator() const;
Rational& operator= (const Rational& rhs);
friend ostream& operator<<(ostream& os, const Rational& rhs);

public:
int num;
int deno;
};

Rational::Rational(int numerator, int denominator)
: num(numerator), deno(denominator)
{
std::cout << "Rational Construct" << std::endl;
}

int Rational::numerator() const
{
std::cout << "numerator: " << num < a(5);
a[1].numerator(); //错误
}

之所以出现错误的原因是：operator[] 返回一个代理对象而不是实际的Rational对象。但成员函数numerator()和denominator()只存在于Rational对象上，而不是其代理对象。

但是我想，为什么编译器在这里不调用隐式转换函数

operator T() const
{
return theArray.array[itemIndex];
}

呢？如果调用了，那么错误也就应该不会出现了

一个简单的解决办法是

int main()
{
Array a(5);
(( Rational ) a[1] ).numerator();
}