Without virtual, you get:

Animal   Animal
  |        |
 dog      cat
    \    /
    dogcat   ← two copies of Animal → ambiguous speak()

With virtual:
   
Animal        ← only one shared copy
   /    \
 dog    cat
    \  /
   dogcat        ← speak() is unambiguous


// Online C++ compiler to run C++ program online
#include <iostream>

class Animal{
    public:
    void speak(){
        std::cout<<"hello"<<std::endl;
    }
};

class dog: virtual public Animal{
    
};

class cat: virtual public Animal{
    
};

class dogcat: public dog, public cat{
    
};

int main() {
    
    dogcat a;
    a.speak();

    return 0;
}