在C ++中覆盖虚函数时，是否可以更改返回类型？ [英] it is possible to change return type when override a virtual function in C++?

问题描述

我遇到了关于覆盖虚拟函数的问题，事实上，它是关于hessian（一个Web服务协议）。

I encounter a problems about override virtual functions, in fact,it is about hessian (a web service protocol).

它有一个基类Object，派生类：Long，Int，String，...，所有派生类都有一个无虚函数value。

it has a base class Object, and some derived classes : Long,Int,String,...,all derived classes has a no-virtual function "value"

   class Object  
   {  
     ...    
   };  


   class Long :public Object  
   {  
       ...  
   public:  
       typedef long long  basic_type;  
       basic_type value(){return value_;}  
   private:  
       basic_type value_;  
       ...  
   };  


   class Int :public Object  
   {  
      ...  
   public:  
       typedef int basic_type;  
       basic_type value(){return value_;}  
   private:   
       basic_type value_;  
       ...  
   };  

现在我想添加一个函数，例如toString，它可以将Object转换为字符串： / p>

now I want to add a function ,say, toString ,which can convert Object to a string:

Object *obj  = ...
cout<<obj->toString();

如果我可以将值函数更改为virtual，我只需要在Object中写一个toString函数，否则，我需要写一个虚函数toString，并在所有派生类中覆盖此函数。

if I can change the value function to virtual ,I only need to write a toString function in Object, else, I need to write a virtual function toString, and to override this functions in all derived classes.

例如

   class Object  
   {  
       virtual Type value(); // It seemed that I can't write a function like this,because the Type is different for different derived classes  


       std::string toString()  
       {  
           some_convert_function(value());  
       }  

   };  

但我不能写虚拟值函数，因为返回值不能覆盖。

but I can't write a virtual value function because of return value can't be override.

这个问题有没有什么好的解决方案？

is there any good solution for this issue?

感谢

推荐答案

对象使用虚拟值函数并覆盖返回类型。但是你可以写一个虚拟的toString和模板编程技巧完成几乎相同的事情。

No, you can't write toString in Object using a virtual 'value' function and override the return type. However you can write a virtual toString and with a template programming trick accomplish almost the same thing.

class Object
{
public:
  virtual std::string toString();
}


template < class ValueType >
class BasicType : Object
{
  public:
  typedef ValueType basic_type;
  basic_type value() { return value_; }

  std::string toString()
  {
    return some_convert_function( value_ );
  }

  private:
  basic_type value_;
}

typedef BasicType<long long> Long;
typedef BasicType<int>       Int;

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