Compiler error when trying to implement visitor design pattern

I'm trying to implement the Visitor Design pattern based on a class diagram from a training material. I've been to this training a while ago and I didn't save the code I implemented there. I am stuck at a compilation error and I can't figure out what I did wrong. Most likely I didn't understood the class diagram for this design pattern. Below I put the entire code and the compilation error I received.

    #define _USE_MATH_DEFINES
    #include <list>
    #include "math.h"

    class Cerc;
    class Dreptunghi;
    class Arie;
    class Perimetru;
    class Operatie;

    class Forma
    {
    public:
        virtual void accept(Operatie * op);
        Forma(void){};
        ~Forma(void){};
    };

    class Cerc : public Forma {
    protected:
        int raza;
    public:
        void accept(Operatie * op){op->vizitare(this);};
        int getRaza(){return raza;};
        Cerc(void):raza(3){};
        ~Cerc(void){};
    };

    class Dreptunghi : public Forma {
    protected:
        int lungime;
        int latime;
    public:
        void accept(Operatie * op){op->vizitare(this);};
        int getLungime(void){return lungime;};
        int getLatime(void){return latime;};
        Dreptunghi(void):lungime(3),latime(3){};
        ~Dreptunghi(void){};
    };

    class Operatie{
    public:
        virtual void vizitare(Cerc* obj) = 0;
        virtual void vizitare(Dreptunghi* obj) = 0;
        Operatie(void){};
        ~Operatie(void){};
    };

    class Arie : public Operatie {
    private:
        double arie;
    public:
        double getResult(){return arie;};
        void vizitare(Cerc* obj){arie = arie + M_PI*obj->getRaza();};
        void vizitare(Dreptunghi* obj){arie = arie + obj->getLungime() + obj->getLatime();};
        Arie(void): arie(0) {};
        ~Arie(void){};
    };

    class Perimetru : public Operatie {
    private:
        double perimetru;
    public:
        double getResult(){return perimetru;};
        void vizitare(Cerc* obj){perimetru = perimetru + M_PI*obj->getRaza()*obj->getRaza();};
        void vizitare(Dreptunghi* obj){perimetru = perimetru + 2*(obj->getLungime() + obj->getLatime());};
        Perimetru(void):perimetru(0){};
        ~Perimetru(void){};
    };

    class Editor{
    private:
        std::list<Forma*> forme;
    public:
        Editor(void){};
        void insert(Forma obj){forme.insert(forme.end(), &obj);};
        double calculeazaArieTotala(){
            Arie arie;

            for(std::list<Forma *>::iterator it = forme.begin(); it != forme.end(); ++it){
                (*it)->accept(&arie);
            }

            return arie.getResult();
        };

        double calculeazaPerimetrulTotal(){
            Perimetru permietru;

            for(std::list<Forma *>::iterator it = forme.begin(); it != forme.end(); ++it){
                (*it)->accept(&permietru);
            }

            return permietru.getResult();
        };
        ~Editor(void){};
    };


    int main(void){
        Editor editor;
        cout << editor.calculeazaArieTotala() << endl;
        cout << editor.calculeazaPerimetrulTotal() << endl;
    }

The compiler error I get:

    visitordesignpattern.cpp(23): error C2027: use of undefined type 'Operatie'
    visitordesignpattern.cpp(23) : see declaration of 'Operatie'
    visitordesignpattern.cpp(23) error C2227: left of '->vizitare' must point to class/struct/union/generic type
    visitordesignpattern.cpp(23) error C2027: use of undefined type 'Operatie'
    visitordesignpattern.cpp(23) see declaration of 'Operatie'
    visitordesignpattern.cpp(23) error C2227: left of '->vizitare' must point to class/struct/union/generic type

Thank you!