[C++] problème char*

#include <iostream>

void convert_2(std::ostream & out,unsigned x){
    unsigned i;
    for(i=0;i<8*sizeof(x);++i){
        out << (x & (1 << (8*sizeof(x) - i - 1)) ? 1 : 0);
    }
    out << std::endl;
}

int main(){
    int x = 27;
    convert_2(std::cout,x);
    return 0;
}

#include <bitset>
using namespace std;

template <int nbBit=1>
class gdentier
{
   private:
            bitset<nbBit> N;
   public :
            inline bool get(const int i)
            {
                 return N[i];
            } 
            inline void set(const int i, const bool b)
            {
                 N[i]=b;     
            }
};

// Dans mon .hpp

            inline void convert_2(std::ostream &); // OK
            inline void convert_16(std::ostream &); // OK
            inline void convert_256(std::ostream &); //OK
            
            friend std::ostream &operator<<(std::ostream &, const gdentier &);

[Warning] friend declaration `std::ostream& operator<<(std::ostream &, const gdentier<nbBit> &)' declares a non-template function
[Warning] (if this not way you intended, make sure the function template has already been declared and add <> after the function name here)-Wno-non-template-friend disables this warning

// Dans Mon .cpp

template <unsigned nbBit>
std::ostream &operator<<(std::ostream & out, const gdentier<nbBit> & g)
{
     if (out==std::cout) g.convert_16(out);
else if (out==std::cerr) g.convert_2(out);  // pour cerr on détaille
else g.convert_256(out); // en particulier pour les fichiers

     return out;
}

// Dans mon main()

    gdentier<8> a(72);
    
    cerr << a << endl; // 01001000
    cout << a << endl; // 4B
    
[Linker error] undefined reference to `operator<<(std::ostream &, gdentier<8u> const&)'
[Linker error] undefined reference to `operator<<(std::ostream &, gdentier<8u> const&)'
Id returned 1 exit status

template <unsigned nbBit>
std::ostream & operator << (std::ostream & out,const gdentier<nbBit> & g){
  if     (out == std::cout) g.convert_16(out);
  else if(out == std::cerr) g.convert_2(out);  // pour cerr on détaille
  else g.convert_256(out); // en particulier pour les fichiers
 return out;
}