过载<运算符和继承类 [英] overloading << operators and inherited classes

问题描述

我有一个基类，然后几个派生类。我想重载<这些派生类的操作符。对于正常的操作符，即'+'，虚函数做的伎俩。我理解的标准约定是声明

I've got a base class and then several derived classes. I would like to overload the "<<" operator for these derived classes. For normal operators, i.e. '+', virtual functions do the trick. What I understand to be the standard convention is to declare

friend ostream& operator<<(ostream& out, MyClass& A);

，然后在类之后定义函数。事先我会认为添加虚拟到上面的定义会使它的工作，但经过一些想法（和我的编译器的错误），我意识到，这没有什么意义。

within my class and then define the function after the class. A priori I would think adding virtual to the above definition would make it work, but after some thought (and errors from my compiler) I realize that doesn't make much sense.

我在一个测试用例上尝试了一个不同的粘连，所有的类成员都是公开的。例如：

I tried a different tack on a test case, where all the class members are public. For example:

class Foo{
 //bla
};

ostream& operator<<(ostream& out, Foo& foo){
  cout << "Foo" << endl;
  return foo;
}

class Bar : public Foo{
 //bla
};

ostream& operator<<(ostream& out, Bar& bar){
  cout << "Bar" << endl;
  return bar;
}

///////////////////////

Bar bar = Bar();
cout << bar << endl; // outputs 'Foo', not 'Bar' 

- 基类操作符<正在被调用而不是派生类操作符。在上面的例子中，如何为派生类调用正确的操作符get？更普遍的是，如果我的类有私有成员我想保护，我如何纠正操作符重载使用friend关键字？

So in some way this is "polymorphism gone bad" -- the base class operator<< is being called rather than the derived class operator. In the above example, how do I make the correct operator get called for the derived class? And more generally, if my class has private members I want to protect, how can I correct the operator overloading while using the friend keyword?

推荐答案

您可以使用虚拟助手函数。这里是一个完全未经测试的例子，所以请理解任何语法错误：

You can use a virtual helper function. Here's a completely untested example, so excuse any syntax mistakes:

virtual ostream& Foo::print(ostream& out) const {
    return out << "Foo";
}

virtual ostream& Bar::print(ostream& out) const {
    return out << "Bar";
}

// If print is public, this doesn't need to be a friend.
ostream& operator<<(ostream& out, const Foo& foo) {
    return foo.print(out);
}

编辑：根据@

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