Everything works fine here

Animal.cpp

#include "Animal.h"
#include <string>
#include<iostream>

Animal::Animal(int age, std::string name) : name(name), age(age) { }


Animal::~Animal() { }

void Animal::sleep(){ printf("Shhh! %s is sleeping...\n", this->name); }

bool Animal::operator==(const Animal &animal){
	return this->age == animal.age;
}

Animal.h

#include <string>

class Animal
{

private:
	int age;
	std::string name;


public:
	Animal(int age, std::string name);
	~Animal();
	void sleep();
	bool operator==(const Animal &animal); // the other animal is referenced by "this"
	int getAge() const { return age; }
	std::string getName() const { return name; }
};

std::ostream &operator<<(std::ostream &o, const Animal &animal){
	return o << "Name: " << animal.getName() << " Age: " << animal.getAge();
}