Overloading << operator in c++?

I'm trying to overload the << operator but I'm getting some errors of this kind:

passing const std::ostream' asthis' argument of `std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits]' discards qualifiers

Here is my code:

#include<iostream>
using namespace std;

class nod{
    protected: 
    int info;
    nod *next;
    friend class lista;
    friend const ostream &operator<<(const ostream &,lista&);

};

class lista
{nod *first, *last;
 public:
 lista()
 {first=new nod;
  last=new nod;
  first=last=NULL;}   
 void insert(int);
 // void remove();
  void afisare();
 nod *get_first(){ return first;};
};

void lista::insert(int x)
{    nod *nou=new nod;    
     nou->info=x;    
     if(!first)
                     first=last=nou;
     else         
                     nou->next=first;  
     first=nou;
     last->next=first;}


const ostream &operator<<(const ostream &o,lista &A)
{nod *curent=new nod;
o<<"Afisare: ";
curent=A.get_first();
if(curent)
          o<<curent->info<<" ";
curent=curent->next;
while(curent!=A.get_first())
          {o<<curent->info<<" ";
          curent=curent->next;}
return o;
}



int main()
{lista A;
A.insert(2);
A.insert(6);
A.insert(8);
A.insert(3);
A.insert(5);
cout<<A;
system("pause");
return 0;}