help on how to reload ostream operator in a template class

eggie · #1 07-Feb-2005, 11:08

Hello everyone,
I am trying to learn C++ by myself. Well, I might be too old to learn a computer language. I need a hand to write a matrix library for a simulation.
My question is how to reload the ostream operator << in a template the class.
I tried the following way, but it didnt work:
1.

CPP / C++ / C Code:

template <class T> class vec {
private:
T* p; int col;
public:
bla bla bal //constructor and destructor
template<class T> friend ostream& operator << (ostream& os, vec<T>& a);
};

template<class T> ostream& operator << (ostream& os, vec<T>& a) {
for (int i= 0; i<a.col ; i++)
os << a.p[i] << "  ";
return os;
}

QED · #2 07-Feb-2005, 11:40

I'd be happy to help, if you could also provide the error message that your compiler issues in response to this code. If there are any other portions of code that might be relevant, such as #include statements, please include those lines as well.

One more note: this forum supports special tags for posting C/C++ code samples, which highlight the languages' syntactical constructs and produce a more readable example.

Like this:

CPP / C++ / C Code:

template <class T> class vec {
private:
  T* p; int col;
public:
  //bla bla bal //constructor and destructor
  template<class T> friend ostream& operator << (ostream& os, vec<T>& a);
};

template<class T> ostream& operator << (ostream& os, vec<T>& a) {
  for (int i= 0; i<a.col ; i++)
    os << a.p[i] << " ";
  return os;
}

To start the code sample, use the tag [c++ ], without the extra space I inserted before the closing brace. To end it, use [/c++ ].

Matthew

eggie · #3 07-Feb-2005, 11:51

Thanks a lot, Matthew.
Here is the code:

CPP / C++ / C Code:

# include <iostream>
using namespace std;

template<class T>
class vec {
 private:
  int col;
  T *p;
 public:
  //constructor
  vec();

 //operator reloaded
  template<class T>  friend  ostream& operator<< (ostream& os, const vec<T>& a);
 };
template <class T>  ostream& operator<< (ostream& os, const vec<T>& a){
  for (int i=0 ; i<a.col ; i++)
    {
      //      double x= a.p[i];
      os << a.p[i]  << "  ";
    }
  return os;
  }

error message:
vec.h:20: error: declaration of `class T'
vec.h:6: error: shadows template parm `class T'

thanks,

QED · #4 07-Feb-2005, 13:32

eggie,

The error message indicates that there is a conflict between the friend declaration

CPP / C++ / C Code:

template<class T> friend ostream& operator<< (ostream& os, const vec<T>& a);

and the earlier line

CPP / C++ / C Code:

template<class T>
class vec {

Some compilers will not complain about your code, but according to the specification for Standard C++, you may not redeclare a template parameter. By typing "template<class T>" again inside the class declaration, you are redeclaring the symbol "T". I don't know if this explanation makes much sense, but one solution is to change your friend declaration to the following:

CPP / C++ / C Code:

friend ostream& operator<< <T> (ostream& os, const vec<T>& a);

This is called a "bound friend declaration," because you are using the same template parameter for the friend function that was used for the class. It is possible to declare unbound friends that have their own independent template parameters, but in your case that would not make sense.

Feel free to ask further questions.

Matthew

P.S. Most often, you will see the term "overloading" used to refer to the type of polymorphism in your code.

eggie · #5 07-Feb-2005, 17:14

Thank you so much, my friend.

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