为什么dynamic_cast是邪恶的?在这种情况下，我应该使用dynamic_cast吗? [英] Why is dynamic_cast evil or not ? Should I use dynamic_cast in this case?

问题描述

有人说 dynamic_cast的使用通常意味着错误的设计和dynamic_cast可以由虚拟函数代替

1. 为什么使用 dynamic_cast 被认为是不良设计?

2. 假设我具有函数名称 func(Animal * animal，int animalType)，在func中的实现类似于:

1. why is the use of dynamic_cast considered bad design?

2. Suppose I have I function name func(Animal* animal, int animalType) , the implementation in func is like:

bool func(Animal* animal, int animalType)
{
  ...
  /* Animal is the base class of Bear, Panda, Fish ....
  dynamic_cast animal to real animals(Bear, Panda, Fish...) 
  according to animalType. Do some processing with this specific
  type of animal, using its additional information beyond base 
  class Animal. */
}

在这种情况下，是否正确使用了 dynamic_cast ?

Is this case a proper use of dynamic_cast?

推荐答案

这完全是使用 dynamic_cast 的错误位置.您应该使用多态.每个 Animal 类都应该具有一个 virtual 函数，例如 process ，在这里您应该只调用 animal-> process().

This is EXACTLY the wrong place to use dynamic_cast. You should be using polymorphism. Each of the Animal classes should have a virtual function, say, process and here you should just call animal->process().

class Animal {
    virtual void Process() = 0;
}

class Cat : public Animal {
    void Process() { std::cout << " I am a tiny cat"; }
}

class Bear : public Animal {
    void Process() { std::cout << "I am a big bear"; }
}

void func(Animal * animal) {
    if (animal != nullptr) { animal->Process(); }
}

其他问题.

如果 animal 是 Dog ，但由于错误导致 animal_type 说它是 Cat ，该怎么办?

What if animal is a Dog, but due to a bug animal_type says its a Cat?

有时需要 static_cast ，并且如果可能的话，请使用它代替 dynamic_cast .动态转换具有静态转换所没有的额外性能成本.为此，您必须确保知道传入的类型，因为 static_cast 更不安全.

There are times when static_cast is necessary, and if possible use it instead of dynamic_cast. Dynamic cast has the additional performance cost that static cast does not. For this, you need to be sure you know the type that is coming in, since static_cast is more unsafe.

至少， animal_type 应该是 Animal 的成员.

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