返回对象的C ++函数

Question

How can I write a function that

returns an object of a different class each time

depending on its parameters ?

ex

class pet{....};
class cat:public pet {.....};
class dog:public pet {.....};
cat_or_dog make pet (int a){
if (a) { cat b; return b;}
else { dog b; return b;}};

Answer 1

I assume you are looking to take advantage of polymorphism.

Your function will need to return a pointer to a pet , preferably a smart pointer like std::unique_ptr<pet> :

#include <memory>

class pet{
public:
  virtual ~pet(){};
};
class cat:public pet {};
class dog:public pet {};

std::unique_ptr<pet> makePet(int a){
  if (a)
    return std::unique_ptr<pet>(new cat);
  else
    return std::unique_ptr<pet>(new dog);
}

int main() {
  auto pet = makePet(2);
}

And your pet should have a virtual destructor in order to clean up properly.

Answer 2

@Chris Drew's answer works if your classes both inherit from the same base class. However, if they don't there is another option. You can use std::optional to optionally return an object. You can use it like this

std::pair<std::optional<cat>, std::optional<dog>> makePet(int a)
{
    std::pair<std::optional<cat>, std::optional<dog>> pet;
    if(a)
        pet.first = cat();
    else
        pet.second = dog();
    return pet;
}

You can then check which pet is there by checking the std::optional within the pair like this

std::pair<std::optional<cat>, std::optional<dog>> pet = makePet(i);

if (pet.first)
{
    //Do things with cat
}
else
{
    //Do things with dog
}

Be warned though that std::optional is still experimental so it might not be implemented by your compiler.