Наследование в С++

[3AKPbIBAKA]

Здравствуйте!

#include <iostream>
using namespace std;
class exp{
	public:
		virtual exp* diff()=0;
		virtual void print()=0;
};
class numb:public exp{
		int val;
public:	numb(int a){val=a;}
		exp* diff(){
			exp* A=new numb(0);
			return A;
		}
		void print(){cout<<val;}
};

class var:public exp{
		char x;
public:	var(char c){x=c;}
		exp* diff(){
			exp* A=new numb(1);
			return A;
		}
		void print(){cout<<x;}
};
class add:public exp{
		exp* val1;exp* val2;
public:	add(exp* a,exp* b){val1=a;val2=b;}
		exp* diff(){
			exp* A=new add (val1->diff(),val2->diff());
			return A;
		}
		void print(){
			cout<<"(";
			val1->print();
			cout<<")+(";
			val2->print();
			cout<<")";
		}
};

class sub:public exp{
	exp* val1;exp* val2;
public:	sub(exp* a,exp* b){val1=a;val2=b;}
		exp* diff(){
			exp* A=new sub (val1->diff(),val2->diff());
			return A;
		}
		void print(){
			cout<<"(";
			val1->print();
			cout<<")-(";
			val2->print();
			cout<<")";
		}
};

class mul:public exp{
	exp* val1;exp* val2;
public:	mul(exp* a,exp* b){val1=a;val2=b;}
		exp* diff(){
			exp* A=new add (&mul(val1->diff(),val2),&mul(val2->diff(),val1));
			return A;
		}
		void print(){
			cout<<"(";
			val1->print();	
			cout<<")*(";
			val2->print();
			cout<<")";
		}
};

class divo:public exp{
exp* val1;exp* val2;
public:	divo(exp* a,exp* b){val1=a;val2=b;}
		exp* diff(){
			exp* A=new divo( &divo( &sub((val1->diff(),val2),&mul(val2->diff(),val1) ), val2 ) , val2);
			return A;
		}
		void print(){
			cout<<"(";
			val1->print();	
			cout<<")/(";
			val2->print();
			cout<<")";
		}
};
class sinus;
class cosus:public exp{
exp* val;
public:	cosus(exp* a){val=a;}
		exp* diff(){
			exp* A=new mul(&sinus( val),val->diff());
			return A;
		}
		void print(){
			cout<<"cos(";
			val->print();
			cout<<")";
		}
};
class sinus:public exp{
exp* val;
public:	sinus(exp* a){val=a;}
		exp* diff(){
			exp* A=new mul(&cosus( val),val->diff());
			return A;
		}
		void print(){
			cout<<"sin(";
			val->print();
			cout<<")";
		}
};
class tg:public exp{
	exp* val;
public:	tg(exp* a){val=a;}
		exp* diff(){
			exp* A=new mul(&divo(&sinus(val),&cosus( val)),val->diff());
			return A;
		}
		void print(){
			cout<<"tg(";
			val->print();
			cout<<")";
		}
};

void main(){
exp *e = new add(new numb(1), new var('x'));
std::cout << "\n";
exp *de = e->diff();
de->print();
cout << "\n";
int o;
cin>>o;
delete e;
delete de;
}

Выдает ошибку

Error	1	error C2440: '<function-style-cast>' : cannot convert from 'exp *' to 'sinus'

Подскажите, пожалуйста, в чем дело.

[russian-stalker]

...
class sinus;
...
class sinus:public exp{
...

[russian-stalker]

...
class sinus;
...
class sinus:public exp{
...

[3AKPbIBAKA]

А как правильно объявить такой класс? Если написать так

...
class sinus:public exp;
...
class sinus:public exp{
...

то появляются ошибки