enum TypeKind
{
    INT,
    DOUBLE,
    BOOL,
    STRING
};

class Type
{
    TypeKind type;
protected:
    Type(TypeKind t)
        : type(t)
    {
    }
public:
    TypeKind getType(void) { return type; }
    virtual ~Type(void) { }
    template<typename T> T getValue(void) { }
};

class Integer : public Type
{
    int value;
public:
    Integer(int v)
        : Type(INT), value(v)
    {
    }
    int getValue()
    {
        return value;
    }
};

// klasy się dorobi...

class Expression
{
public:
    virtual Type* eval(void) = 0;
    virtual ~Expression(void) { }
};

class AddOp : public Expression
{
    Expression *left, *right;
public:
    //...
    virtual Type *eval(void)
    {
        Type *l = left->eval(), *r = right->eval();
        if(l->getType() == INT)
        {
            return Integer(dynamic_cast<Integer*>(l)->getValue() + dynamic_cast<Integer*>(r)->getValue());
        }
        //...
    }
};