VC6.0中重载操作符函数无法访问类的私有成员

在 C++ 中,操作符(运算符)可以被重载以改写其实际操作。同时我们可以定义一个函数为类的朋友函数(friend function)以便使得这个函数能够访问类的私有成员,这个定义通常在头文件中完成。

在Visual C++中定义一般的函数为朋友函数通常是没有问题的。然而对某些重载操作符的函数,即使我们将它们定义为类的朋友函数,VC的编译器仍然会显示出错信息,认为这些朋友函数无权访问类的私有成员。我认为这应该是VC6.0的bug。

以下代码就是个例子:

// 头文件 "Sample.h"
#include<iostream>
using namespace std;

class Sample {
public:
	Sample();
	friend ostream &operator<<(ostream &out, const Sample s);
	friend istream &operator>>(istream &in, Sample & s);

private:
	int x;
};

// 实现文件 "Sample.cpp"
#include "Sample.h"

Sample::Sample() {
	x=0;
}

istream &operator>>(istream &in, Sample & s) {
	cout<<"Please enter a value"<<endl;
	in >> s.x ;
	return in;
}

ostream &operator<<(ostream &out, const Sample s) {
	cout << s.x << endl;
	return out;
}

以上代码在gnuc++中编译运行毫无问题。但是在VC++6.0中编译的时候就会出现以下的编译错误:

Compiling…
Sample.cpp
c:\temp\sample.cpp(8) : error C2248: ‘x’ : cannot access private member declared in class ‘Sample’
c:\temp\sample.h(19) : see declaration of ‘x’
c:\temp\sample.cpp(13) : error C2248: ‘x’ : cannot access private member declared in class ‘Sample’
c:\temp\sample.h(19) : see declaration of ‘x’
Error executing cl.exe.Sample.obj – 2 error(s), 0 warning(s)

在VC++ 6.0中解决这个问题有以下几种方法:

  • 在头文件中实现作为朋友函数的操作符函数的重载,也就是说在实现文件”Sample.cpp”中将函数重载的实现去掉,而将头文件修改如下:
    // 修改后的头文件 1 "Sample.h"
    #include<iostream>
    using namespace std;
    
    class Sample {
    public:
    	Sample();
    	friend ostream &operator<<(ostream &out, const Sample s);
    	friend ostream &operator<<(ostream &out, const Sample s) {
    	    cout << s.x << endl;
    	    return out;
    	}
    
    	friend istream &operator>>(istream &in, Sample & s);
    	friend istream &operator>>(istream &in, Sample & s) {
    		cout<<"Please enter a value"<<endl;
    	    in >> s.x ;
    	    return in;
    	}
    private:
    	int x;
    };
    
    
  • 在头文件中类定义之前将类和朋友操作符函数的原型特别声明一下,也就是将头文件修改如下(实现文件”Sample.cpp”不用作任何修改):
    // 修改后的头文件 2 "Sample.h"
    #include<iostream>
    using namespace std;
    
    // 以下3行代码为新加入
    class Sample;
    ostream &operator<<(ostream &out, const Sample s);
    istream &operator>>(istream &in, Sample & s);
    
    class Sample {
    public:
    	Sample();
    	friend ostream &operator<<(ostream &out, const Sample s);
    	friend istream &operator>>(istream &in, Sample & s);
    
    private:
    	int x;
    };
    
    
  • 第三种方法是对I/O名空间的使用实行明确声明,也就是说在头文件”Sample.h”中直接写:
    #include<iostream>
    using std::ostream;
    using std::istream
    ….
    取代 “using namespace std;”
    注意:在这个例子里我们在实现文件 “Sample.cpp”中包含 “using namespace std;”这句话,否则在实现中就不能使用 “cout” , “cin”, “<< “, “>>” 和 endl 这些关键字和符号。修改后的完整代码如下:

    // Sample.h
    #include<iostream>
    
    using std::istream;
    using std::ostream;
    
    class Sample {
    public:
    	Sample();
    	friend ostream &operator<<(ostream &out, const Sample s);
    	/*friend ostream &operator<<(ostream &out, const Sample s) {
    	    cout << s.x << endl;
    	    return out;
    	}*/
        friend istream &operator>>(istream &in, Sample & s);
    	/*friend istream &operator>>(istream &in, Sample & s) {
    		cout<<"Please enter a value"<<endl;
    	    in >> s.x ;
    	    return in;
    	}*/
    private:
    	int x;
    };
    // "Sample.cpp"
    #include "Sample.h"
    using namespace std;
    
    Sample::Sample() {
    	x=5;
    }
    istream &operator>>(istream &in, Sample & s) {
    		cout<<"Please enter a value"<<endl;
    	    in >> s.x ;
    	    return in;
    	}
    
    ostream &operator<<(ostream &out, const Sample s) {
    	cout << s.x << endl;
    	return out;
    }
    
    

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