ISO C++ says that these are ambiguous,

Question

I have to overload the shift operator " << " both for writing in console and to write on a binary file..

I am doing okay for the ostream overloading, while I am having some problem overloading the fstream, here it is:

in my header:

friend ostream &operator<<(ostream &, const Fotografia &);
friend fstream &operator<<(fstream &, const Fotografia &);

in my cpp file:

fstream &operator<<(fstream & miofile, const Fotografia & sorgente)
{
        //Open the file
        miofile.open("data.dat", ios::binary | ios::app);
        if(!miofile) cerr << "Can't open the file\n";
        miofile << strlen(sorgente.Titolo);
        miofile << endl;        
        miofile << sorgente.Titolo;
        //I close the file
        miofile.close();
        return miofile;

}

Here's the error I am facing:

In function `std::fstream& operator<<(std::fstream&, const Fotografia&)':

ISO C++ says that these are ambiguous, even though the worst conversion for the first is    better than the worst conversion for the second:

std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*) [with _Traits = std::char_traits<char>] 

std::fstream& operator<<(std::fstream&, const Fotografia&) 

What I understood so far is that there's ambiguosity between the overloaded function I just created and the standard fstream << . Now, what I do not understand is why, because my overloaded function should work just for the class "Fotografia" (which was created by me), while I am trying to write a char * .

I thought I could solve this problem by calling the fstream operator with the "::" scope but I am not sure.

Could anyone help me out here please? :)

EDIT:

I am posting the code for the header and the code for the constructor

         //Costruttore,distruttore,costruttore di copia,operatore di assegnazione.
         Fotografia(char * titolo = "Untitled" , char * formato = ".jpeg");
         ~Fotografia() { delete [] Titolo; delete [] Formato;}
         Fotografia(const Fotografia &);
         Fotografia &operator=(const Fotografia &);

This is in the cpp:

 Fotografia::Fotografia(char * titolo , char * formato)
 {
     Titolo = new char[strlen(titolo)+1];
     strcpy(Titolo,titolo);
     Formato = new char[strlen(formato)+1];
     strcpy(Formato,formato);
  } //Fine costruttore

Answer 1

Get rid of the operator char* in Fotografia, or mark it explicit.

Also, instead of limiting the code to inserting in an fstream, you could insert into an arbitrary basic_ostream. That would still work for an fstream, and would give you more flexibility to use other forms of output streams. That would also eliminate the error.

Answer 2

It makes no sense to overload operator<< for std::fstream. First, because experienced C++ programmers hardly every use std::fstream; they use std::ofstream or std::ifstream. And second, because once you've used a <<, the return value is a std::ostream anyway, so the operator<< for ofstream would never be called.

Of course, your implementation of the operator also violates all of the rules. You don't open or close a file in the operator; the operator is for formatting the data. If you want to support two different formats, the usual way would be to define a manipulator to choose between them, and let the client decide. (See std::ios_base::xalloc() and company for where to put the additional state.)