error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'void' (or there is no acceptable conversion)

Question

I'm not sure what the problem is. It gives me the error:

error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'void' (or there is no acceptable conversion)

I overloaded the << so it shouldn't be giving me this error, right?

#ifndef ANIMAL_H_
#define ANIMAL_H_ 
#include <iostream> 
#include <string>
using namespace std;

static int counter;
static int getAnimalCount() { return counter; }

class Animal {
    protected:
       string *animalType;
    public:
       virtual void talk() = 0;
       virtual void move() = 0;
       string getAnimalType() { return *animalType; }

               //PROBLEM RIGHT HERE V

       friend ostream& operator<<(ostream&out, Animal& animal) {
           return out << animal.getAnimalType() << animal.talk() << ", " << animal.move();
       };
       ~Animal() {
           counter--;
           animalType = NULL;
       }
 };
 class Reptile : public Animal { 
     public:
         Reptile() { animalType = new string("reptile"); };
 };
 class Bird : public Animal {
     public:
         Bird() { animalType = new string("bird"); };
 };
 class Mammal : public Animal{
     public:
         Mammal() { animalType = new string("mammal"); };
 };
 #endif /* ANIMAL_H_ */

Answer 1

virtual void talk() = 0; specifies a function with return type void. This means it does not return anything. The same happens when you define Animal::move as virtual void move() = 0;.

out << animal.getAnimalType() << animal.talk() << ", " << animal.move(); tries to print the result of animal.talk() and the result of animal.move() - neither of which exists (remember, neither talk() nor move() return any value!)