I want a derived class to be able to copy from another class if it has the same type, if not just do nothing.

Example in semi-pseudocode:

class Animal
{
public:
  virtual void CopyFrom(Animal* animal) = 0;

  // TODO: virtual destructor
};

class Dog : Animal
{
public:
  Dog() {}

  void CopyFrom(Animal* animal) override {
    if (animal is Dog) {
      likes_to_bark = ((Dog)animal).likes_to_bark;
    }
  }

private:
  bool likes_to_bark;
}

I know this can be achieved a dynamic cast, but that is considered code smell. I was looking into the visitor pattern and double dispatch, but I could not find a way to do this. Any ideas?

CodePudding user response：

 if (animal is Dog)

C has a solution for you: RTTI. Unless disabled, it allows dynamic_cast to cast a down-pointer (to parent type) to an up-pointer (to derived type) and returns a null pointer if the cast isn't possible.

if (dynamic_cast<Dog*>(animal)) {
    // this bloc is only executed if animal is indeed a Dog
}

Additionally, you could tweak and rename your function copyFrom to operator=, allowing nice user syntax:

Dog& operator=(Dog const& other) = default // or define it if it has specificities
    
Dog& operator=(Animal const& animal)
{
    Dog const* other = dynamic_cast<Dog const*>(&animal)
    if (other) {
        *this = *other;
    }
    return *this;
}

int main()
{
    Animal spider;
    Dog pluto, laika;
    Animal& animal = pluto;

    laika = spider; // does nothing
    laika = animal; // copies pluto to laika
}

I know this can be achieved a dynamic cast, but that is considered code smell.

A code-smell is what it is: a smell. I'd rather see a simple code using dynamic_cast in a proper manner that something trying to hide it uses casting under the hood in a convoluted way.
The tool is available, it can be used. Just avoid abusing it.

CodePudding user response：

rtti is of course necessary but you can achieve this readily in the base:

class Animal
{
public:
  virtual void CopyFrom(Animal* animal){
     if(typeid(this) == typeid(animal){
         *this = *animal;
     }
  }
};