在C ++中继承和覆盖ostream运算符 [英] Inheriting and overriding ostream operator in C++

问题描述

我一直在尝试找到一个答案，但没有人会有和我一样的问题。

我使用几个派生类。 ostream运算符<<对于每一个都应该打印出一些共同的东西，每一个，和一些事情具体到每个。后来，我想从这些派生类进一步派生，再次新的派生类需要打印出一些在它们之上的世代的东西。

例如：

基本类.h文件

  class Base 
 
 {
 
 
 
 int FirstClassNumber; 
 
 //我目前使用的声明，朋友给了我
 //我很确定我的问题在这里。 
 
 
 public：
 
 friend ostream&运算符<< （ostream& os，const Base& base）
 {
 base< os; 
 
 return os; 
} 
 
 virtual void operator<< （ostream& os）const = 0; 
 
}; 
  

Base.cpp文件包括以下行：

  void Base :: operator<< （ostream& os）
 {
 os<< FirstClassNumber; 
} 
  

然后我派生：（FirstDerived.h）

  class FirstDerived：public Base 
 
 {
 
 int SecondClassNumber; 
 
}; 
  

FirstDerived.cpp

：

  FirstDerived :: operator<< （ostream& os）
 {
 os<< 
 
第一个数字是：
 
 //这是不工作的行 - 别人给我这个语法
 
< < Base :: operator<<< 
 
<<第二个数字是
 
<< SecondClassNumber; 
} 
  

然后我想导出：

 code> class SecondDerived：Public FirstDerived 
 {
 
 int ThirdClassNumber; 
 
}; 
  

Second.cpp：

  FirstDerived :: operator< ;（ostream& os）
 {
 os << 
 
 FirstDerived :: operator<< 
 
<< number是
 
<< ThirdClassNumber; 
 
} 
  

我认为问题很可能是在程序的最开始的声明，或者像 Base :: operator<< 。 p>

另一个可能性是我不会在每个继承类的.h文件中重新声明它。我应该是，如果是，我应该使用什么语法？

建议我使用 static_cast 方法，但我的教授因此不会给我们太多的帮助）说，有一个更好的方法来做到这一点。任何建议？

解决方案

一个简单的方法是：

  class Base 
 {
 int FirstClassNumber; 
 
 public：
 virtual void serialize（ostream& os）const 
 {
 os< FirstClassNumber; 
} 
}; 
 
 //为基类实现流运算符。 
 //所有它是调用erialize这是一个虚拟方法
 //将调用最导出的版本。 
 ostream&运算符<< （ostream& os，const Base& base）
 {
 base.serialize（os）; 
 
 return os; 
} 
 
 class FirstDerived：public Base 
 {
 int SecondClassNumber; 
 
 public：
 //覆盖serialize以使其调用基本版本。 
 //然后输出任何本地数据。 
 virtual void serialize（ostream& os）const 
 {
 Base :: serialize（os）; 
 os<< SecondClassNumber; 
} 
}; 
  

I've been trying to find an answer to this, but no one seems to have exactly the same problem as I do.

I am working with several derived classes. The ostream operator << for each of these should print out some things common to each, and some things specific to each. Later on, I would like to further derive from these derived classes, and again the new derived classes need to print out some things that are in the "generations" above them.
For example:

The Base class .h file

class Base

{  



 int FirstClassNumber;

//The declaration I'm currently working with, that a friend gave me
//I'm pretty sure my problem lies here.


public:

friend ostream& operator << (ostream& os, const Base &base)
{
    base << os ;

    return os;
}

virtual void operator << (ostream& os) const = 0;

};

The Base.cpp file includes these lines:

void Base::operator << (ostream& os)
{
  os << FirstClassNumber;
}

Then I derive: (FirstDerived.h)

class FirstDerived : Public Base

{ 

int SecondClassNumber;

};

FirstDerived.cpp:

FirstDerived::operator << (ostream& os)
{
  os <<

  "The first Number is:

 //This is the line that isn't working - someone else gave me this syntax

  << Base::operator<< 

  << "The second number is"

  << SecondClassNumber;
}

Then I want to derive:

class SecondDerived: Public FirstDerived
{ 

int ThirdClassNumber;

};

Second.cpp:

FirstDerived::operator << (ostream& os)
{
  os <<

 FirstDerived::operator<<

 << "The third number is "

 << ThirdClassNumber;

 }

I think the problem is most likely either the declaration in the very start of the program, or the lines like Base::operator<<.

Another possibility is that I'm not redeclaring it in the .h file of every inherited class. Should I be, and if so what syntax should I use?

It was suggested to me to use the static_cast method, but my professor (the one who wrote the assignment, and therefore won't give us too much help with it) said that there's a better way to do it. Any suggestions?

解决方案

A simple technique for this is:

class Base
{  
    int FirstClassNumber;

    public:
        virtual void serialize(ostream& os) const
        {
             os << FirstClassNumber;
        }
};

// Implement the stream operator for the base class.
// All it does is call erialize which is a virtual method that
// will call the most derived version.
ostream& operator << (ostream& os, const Base &base)
{
    base.serialize(os);

    return os;
}

class FirstDerived:public Base
{  
    int SecondClassNumber;

    public:
        // Override serialize to make it call the base version.
        // Then output any local data.
        virtual void serialize(ostream& os) const
        {
             Base::serialize(os);
             os << SecondClassNumber;
        }
};

这篇关于在C ++中继承和覆盖ostream运算符的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！