多态 C++ 参考 [英] polymorphic C++ references

问题描述

我想知道如何使用引用而不是指针来实现多态性.

I was wondering how you can do polymorphism with references, as opposed to pointers.

为了澄清，请参见以下最小示例:

To clarify, see the following minimal example:

class A;

class B {
  public:
    A& a; ///////////////// <- #1
    B();
    void doStuff();
};

class A {
  public:
    virtual void doSmth() = 0;
};
void B::doStuff() {
  a.doSmth();
}

class A1 : public A {
  public:
    void doSmth() {
    }
};

B::B() : a(
    *        ////////////// <- #2
      (new A1)  /////////// <- #3
     ) {
}

这可以编译并且可以工作，但是这里最重要的一点是 #1 行中的 a 是一个引用，所以为了能够多态地使用它(这是一个实际的词吗?)，如 #3 行所示，我必须将指针转换为引用"；通过取消引用它.

This compiles and works, but as the most important point here is that a in line #1 is a reference, so in order to be able to use it polymorphically (is that an actual word?), as shown in line #3 I have to "convert a pointer to a reference" by dereferencing it.

这让我觉得有点奇怪，我想知道是否有更好的(在 cleaner 的意义上)方法.只有我吗?

This strikes me as a bit odd, and I was wondering if there is a better (in the sense of cleaner) way. Is it just me?

如果我根本不需要 new 那就太好了，但是在声明 (!) B 时，我不知道如何创建 A1 (!) as A 是前向声明 - A1 与 B 在相同的编译单元中实现.不过，在这种情况下是否真的需要动态内存分配?你会怎么做呢?

It would be great if I didn't need a new at all, but when declaring (!) B I have no clue how to create an instance of A1 (!) as A is a forward declaration -- A1 is implemented in the same compilation unit as B. Still, is there a real need for dynamic memory allocation in this case? How would you do this?

抱歉，有点双重问题.

注意:B 很大(我不能为它创建一个模板类)，并且会在程序终止时恰好超出范围——a 很小并使两个大模块相互通信，只要 B 的实例存在(只有一个)就需要它.

Note: B is huge (and I cannot make a template class of it), and will go out of scope precisely when the program terminates -- a is small and makes two big modules talk to each other, it will be needed as long as the instance of B lives (there is only one).

我刚刚意识到，由于 A 和 B 都是有效的单例，我可以简单地创建 A1 的 static 实例 在B 的编译单元中，避免动态内存分配(即使有两个B 也可以轻松使用相同的A<实例/代码>).公平地说，我没有将此作为答案发布，但会接受促使我提出此解决方案的答案.

I just realised, that since both A and B are effectively singletons, I can simply create a static instance of A1 in the compilation unit of B, avoiding dynamic memory allocation (even if there were two Bs they could easily use the same instance of A). To be fair, I did not post this as answer, but will accept the answer that prompted me to come up with this solution.

推荐答案

没什么奇怪的.多态性适用于指针 和 引用:

There's nothing odd. Polymorphisms works both for pointers and references:

struct Base { };
struct Derived : Base;

void foo(Base &);

int main() {
  Derived x;
  foo(x);    // fine
}

您将此与另一个问题混为一谈，即创建对动态对象的引用:

You're conflating this with another issue, namely creating a reference to a dynamic object:

T * pt = new T;
T & rt = *pt;

T & x = *new T;  // same effect

请注意，通过引用仅跟踪动态对象通常是非常糟糕的样式，因为删除它的唯一方法是通过 delete &x;，它是很难看出 x 需要清理.

Note that it's generally very bad style to track a dynamic object only by reference, because the only way to delete it is via delete &x;, and it's very hard to see that x needs cleaning up.

您的设计有两种直接的选择:1) 使 a 成为 B 中的成员对象，或 2) 使 a 成为 shared_ptr 或 unique_ptr 并将初始化器更改为 a(new A1).这完全取决于您是否真的需要多态行为，即您是否有 B 的其他构造函数将不同的派生类分配给 a 而不是 A1.

There are two immediate alternatives for your design: 1) make a a member object in B, or 2) make a a shared_ptr<A> or unique_ptr<A> and change the initalizer to a(new A1). It all depends on whether you actually need the polymorphic behaviour, i.e. if you have other constructors for B which assign a different derived class to a other than A1.

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