C++:wikibook 中关于重载指针/引用相关运算符的代码无法编译

Question

我正在尝试 wikibooks ( http://en.wikibooks.org/wiki/C%2B%2B_Programming/Operators/Operator_Overloading ) 中的一段示例代码，但它无法在 Visual Studio 中编译。该代码是关于重载地址、引用和指针运算符(operator&()、operator*() 和 operator->()) :

//file example.cpp
#include "stdafx.h"
#include <iostream>

class T {
public:
    const void memberFunction() const {
        std::cout << "Hello!\n";
    }
};

// forward declaration
class DullSmartReference;

class DullSmartPointer {
private:
    T *m_ptr;
public:
    DullSmartPointer(T *rhs) : m_ptr(rhs) {};
    DullSmartReference operator*() const {
        return DullSmartReference(*m_ptr);
    }
    T *operator->() const {
        return m_ptr;
    }
};

class DullSmartReference {
private:
    T *m_ptr;
public:
    DullSmartReference (T &rhs) : m_ptr(&rhs) {}
    DullSmartPointer operator&() const { // error C2027: use of undefined type 'DullSmartReference'
        return DullSmartPointer(m_ptr);
    }
    // conversion operator
    operator T() { return *m_ptr; }
};


int _tmain(int argc, _TCHAR* argv[])
{
    DullSmartPointer dsp(new T);
    dsp->memberFunction(); // calls T::memberFunction

    T t;
    DullSmartReference dsr(t);
    dsp = &dsr;
    t = dsr; // calls the conversion operator

    std::cin.get();

    return 0;
}

Visual Studio 总是报告编译器错误 C2079。错误信息是

1>e:\projects\bad\example.cpp(20): error C2027: use of undefined type 'DullSmartReference'
1>          e:\projects\bad\example.cpp(13) : see declaration of 'DullSmartReference'
1>e:\projects\bad\example.cpp(21): error C2440: '' : cannot convert from 'T' to 'DullSmartReference'
1>          Source or target has incomplete type

如何修复错误？我正在使用 VS 2010，但我认为版本不重要。谢谢!

更新: 1. 我更新了代码并完成了错误信息。

Answer 1

该示例中的错误(至少在给定的情况下)是 DullSmartReference 在定义之前被使用。在 C2079 行，它试图构造并返回一个 DullSmartReference，如果没有它的定义，这是不可能的。要解决此问题，该部分代码应为:

// forward declaration
class DullSmartReference;

class DullSmartPointer {
private:
    T *m_ptr;
public:
    DullSmartPointer(T *rhs) : m_ptr(rhs) {};
    DullSmartReference operator*() const;   // DullSmartReference not used yet
    T *operator->() const {
        return m_ptr;
    }
};

// DullSmartReference definition goes here

DullSmartReference DullSmartPointer::operator*() const {
    return DullSmartReference(*m_ptr);  // OK; we have the definition by now
}