是否使用继承和多态性重载&#39；&lt；&lt；&#39；？ [英] overloading '<<' with inheritance 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拥有重载的<；<；运算符，而将这些对象存储在Base*的向量中。

至于我要实现的目标；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。

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，但它不会达到我的目的。

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

推荐答案

Make your virtual methods private to separate how an object is used from how its behavior can be customized by derived classes.

这里有一个类似于其他答案的解决方案，但虚拟方法是私有的：

#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 << "
"; } // use << inside class
    ~DerivedOne() { std::cerr << "bye " << *this << "
"; }
  };
}

示例

#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

hi One
One 
bye One

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