Classes derived from the same base

[arczi w]

Suppose I have the following code:

class animal
{
    public: bool is_male; long double strength;
};
class fish : public animal{};
class bear : public animal{};

Later in the file, I created this:

animal * river[_river];
river[0] = new fish;
river[1] = new bear;

I need to find something that would allow me to compare river[0] and river[1] and, in this particular example, yield a falsehood since river[0] is a fish and river[1] is a bear. Is there anything I could do?

[Repeater]

since river[0] is a fish and river[1] is a bear.

Actually, in your code above, river[0] is a pointer-to-animal and river[1] is also a pointer-to-animal. They're both exactly the same type.

Does this do what you're after, though? I am comparing the objects being pointed to, rather than the pointers.

#include <iostream>
#include  <typeinfo> 

class animal
{
public: bool is_male; long double strength;

  virtual inline bool operator==(const animal& other)
  {  
    std::cout << "compa" << ' ';
    return (typeid(*this) == typeid(other));
  }
};

class fish : public animal{};
class bear : public animal{};

int main()
{
  animal * river[2];
  river[0] = new fish;
  river[1] = new bear;

  std::cout << (1 == 2) << '\n';
  std::cout << (*river[0] == *river[0]) << '\n';
  std::cout << (*river[0] == *river[1]) << '\n';
  std::cout << (*river[1] == *river[0]) << '\n';
  std::cout << (*river[1] == *river[1]) << '\n';
  
}

There's something about what you're trying to do here that just feels wrong, though. Feels like you're not working with the language. What's you actual design aim here? Maybe there's a better, more axiomatic C++ way to do it.

[arczi w]

The program seems to do what I want it to. Can you explain why I need to state "virtual inline" before the comparison operator declaration to achieve the result?

[Repeater]

The class needs to be polymorphic - at least one virtual function - in order for this to work. It's not that the comparison operator itself needs to be virtual; that was just convenient. This also works:

#include <iostream>
#include  <typeinfo> 

class animal
{
public: bool is_male; long double strength;

  virtual void dummy(){};

  inline bool operator==(const animal& other)
  {  
    std::cout << "compa" << ' ';
    return (typeid(*this) == typeid(other));
  }
};

class fish : public animal{};
class bear : public animal{};

int main()
{
  animal * river[2];
  river[0] = new fish;
  river[1] = new bear;

  std::cout << (1 == 2) << '\n';
  std::cout << (*river[0] == *river[0]) << '\n';
  std::cout << (*river[0] == *river[1]) << '\n';
  std::cout << (*river[1] == *river[0]) << '\n';
  std::cout << (*river[1] == *river[1]) << '\n';
  
}

but if the dummy function has its virtual qualifier removed, it stops working. What the exact method of calculating what type a polymorhpic object is, I couldn't tell you (and I believe it's implementation dependent), but I believe that the vtable is part of it, so a vtable is necessary, and having a virtual function gets us that vtable.

[arczi w]

Thank you for your help! Can you further explain why the class needs to be polymorphic?