why this dont work? please help me

hi,
I'm studing the + and the = operator overloading to make something like obj1 = obj2 + obj3, but my code wont run here is it:
please see the main program at the botton

Code:

//class definition
class NRacionales{
   friend ostream &operator<<(ostream &, const NRacionales   &);
   public:   
      
      NRacionales(int = 1, int = 1);
      NRacionales operator+(const NRacionales &);
      NRacionales &operator=(NRacionales &);
      .
      .
   private:
      int numerador;
      int denominador;
         
}

//function definitions
//definition of << overloading not need it here

NRacionales::NRacionales(int num, int den)
{
     numerador = num;
     denominador = den;
}

//+ operator overloaded
NRacionales NRacionales::operator+(const NRacionales &derecha)
{
      //returns an instance of the class
       return NRacionales((numerador * derecha.denominador) + 
	 (derecha.numerador * denominador),			  (denominador * derecha.denominador));
}

//= operator overloaded
NRacionales &NRacionales::operator=(NRacionales &derecha)
{
       numerador = derecha.numerador;   
      denominador = derecha.denominador;
      return *this;
}

//main
int main()
{
        NRacionales n1(8,4);
        NRacionales n2(9,3);
        NRacionales n4;
      
        //this block of code works fine
        cout << "N1= " << n1 << endl;
        cout << "N2= " << n2 << endl; 
        cout << "N4= " << n4 << endl;
   
        //this line of code works fine
        cout << "\nn1 + n2 = " << n1 + n2 << endl;
   
      //this dont work , why? 
      n4 = n1 + n2;
      cout << "\nn4 = n1 + n2 = " << n4 << endl;
      system("pause");
      return 0;
}

ok here is the error message the compiler generates(dev c++):

28 C:\programas c++\ejer8_17\main.cpp
no match for `NRacionales& = NRacionales' operator

28 C:\programas c++\ejer8_17\fracciones.h
candidates are: NRacionales&  NRacionales::operator=(NRacionales&)

please excuse my poor english

do you have a default constructor? the line:

NRacionales n4;

attempts to call the default constructor. Since you already have a non-default non-copy constructor the compiler won't provide a default constructor for you.

Since numerado and denominador are private members of NRacionales, when trying to access them for derecha you will need accessor functions as well.

i have the copy constructor:
NRacionales(const NRacionales &);

how about a default constructor:

NRacionales() : numerador(0), denominador(0) {}

or some such beast?