illegal use of type 'void'怎么改正

#include<iostream>
using namespace std;

class Base1{
public:
    virtual void display() const=0;
    virtual void food() const=0;
};
class Base2:public Base1{
public:
    void display() const;
    void food() const;
};

void Base2::display() const{
    cout<<"第一层的是动物"<<endl;
}
void Base2:food() const{
}

class Base3:public Base2{
public:
    void display() const;
    void food() const;
};

void Base3::display() const{
    cout<<"第二层的有动物下的猴子，猫科和鸟"<<endl;
}
void Base3::food() const{
    cout<<"猴子一般都是吃水果的"<<endl;
}

class Base4:public Base3{
public:
    void display() const;
    void food()  const;
};
void Base4::display() const{
    cout<<"第三层中有猫科之下的狮子和老虎，鸟之下的麻雀"<<endl;
}
void Base4::food() const{
    cout<<"狮子和老虎都是吃肉的，而麻雀是吃素的"<<endl;
}

void fun(Base1 *ptr){
    ptr->display();
}
void fun1(Base1 *p){
    p->food();
}

int main(){
    Base2 base2;
    Base3 base3;
    Base4 base4;
    fun(&base2);
    fun1(&base2);
    fun(&base3);
    fun1(&base3);
    fun(&base4);
    fun1(&base4);

    return 0;
}