重载'<<'有不和谐和多态？ [英] overloading '<<' with inhertiance and polymorphism?

问题描述

以下是代码外观的粗略示例，问题是如何让DerivedOne和DerivedTwo重载<<运算符，但将这些对象存储在Base *的向量中。

The following is a rough sample of what the code looks like, the question is how can I have DerivedOne and DerivedTwo have a overloaded << operator but store these objects in a vector of Base*.

至于我想要实现的目标;我希望能够遍历对象向量并输出我在DerivedOne和DerivedTwo中告诉它的信息。

As for what I want to achieve; I want to be able to loop through the objects vector and output the information that I tell it to in the DerivedOne and DerivedTwo.

vector<Base*> objects;

class Base
{
 private:
 object Data
 public:
 object getData() { return Data; }
};

class DerivedOne : public Base
{
}

class DerivedTwo : public Base
{
}

现在我知道有这个，但它不能用于我的目的。

Now I know there is this, but it wont work for my purposes.

friend ostream &operator<<(ostream &stream, Object object)
{
    return stream << "Test" << endl;
}

推荐答案

将您的虚拟方法设为私有，以便将对象的使用方式与派生类的行为自定义区分开来。

这与其他答案解决方案类似，但虚拟方法是私有的：

Here's a similar to other answers solution but the virtual method is private:

#include <iostream>

namespace {
  class Base {
    // private (there is no need to call it in subclasses)
    virtual std::ostream& doprint(std::ostream&) const = 0;
  public:
    friend std::ostream& operator << (std::ostream& os, const Base& b) {
      return b.doprint(os); // polymorphic print via reference
    }

    virtual ~Base() {} // allow polymorphic delete
  };


  class DerivedOne : public Base {
    std::ostream& doprint(std::ostream& os) const {
      return os << "One";
    }
  public:
    DerivedOne() { std::cerr << "hi " << *this << "\n"; } // use << inside class
    ~DerivedOne() { std::cerr << "bye " << *this << "\n"; }
  };
}

示例

Example

#include <memory>
#include <vector>

int main () {
  using namespace std;
  // wrap Base* with shared_ptr<> to put it in a vector
  vector<shared_ptr<Base>> v{ make_shared<DerivedOne>() };
  for (auto i: v) cout << *i << " ";
  cout << endl;
}

输出

Output

hi One
One 
bye One

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