前向声明后类型类的使用无效

我写了这个程序来演示C++中的双重调度。但它显示类型类的使用无效，即使我已经向前声明了我的类。有没有办法在不写入单独的头文件的情况下纠正此问题。

#include <iostream>
#include <string>
using namespace std;
class Number;       
class Integer;
class Rational;
class Number{
    public:
        int num, den;
        virtual void add(Number&) = 0;
        virtual void addInteger(Integer&) = 0;
        virtual void addRational(Rational&) = 0;
        virtual string toString() = 0;
};

class Rational : public Number{
        void addInteger(Integer& i){
            this->num = this->num + i.num*this->den;
            this->den = this->den;
            cout << this->num << "/" << this->den;
        }
        void addRational(Rational &r){
            this->num = this->num*r.den + this->den*r.num;
            this->den = this->den*r.den;
            cout << this->num << "/" << this->den; 
        }
    public:
        void add(Number& n){
            n.addRational(*this);
        }
        Rational(int n, int d){
            this->num = n;
            this->den = d;
        }
};

class Integer : public Number{
        void addInteger(Integer& i){
                this->num += i.num;
                this->den = 1;
                cout << this->num;
        }
        void addRational(Rational& r){
            this->num = this->num*r.den + r.num;
            this->den = r.den;
            cout << "this->num" << "/" << this->den;
        }
    public:
        void add(Number& n){
            n.addInteger(*this);
        }
        Integer(int n){
            this->num = n;
            this->den = 1;
        }
};

int main(){
    cout << "Helo World";
    return 0;
}