Re: Template friend function not compiling (full working code example)

From:

Paavo Helde <myfirstname@osa.pri.ee>

Newsgroups:

comp.lang.c++

Date:

Thu, 22 Jan 2015 15:07:21 -0600

Message-ID:

<XnsA42AEB373E26Emyfirstnameosapriee@216.196.109.131>

JiiPee <no@notvalid.com> wrote in news:DYcww.293940$Ud7.288631@fx11.am4:

Sorry for last time not providing full code. Now I paste here the full
code so you can test the error yourself. I have GCC 4.8, debug build,
C++11 on, Windows XP. Error message: "error: declaration of
'operator*' as non-function".

So I tried to add a simple multiplication function as a friend. And it
to be template function. But it conflicts with the other
multiplication function.

It works if I commend out the other * operator (see the code).
Any idea how to fix this? thanks

Full console based code which does not compile:

#include <iostream>

using namespace std;

template <typename T> class Vector2D2;
template <typename T> Vector2D2<T> operator * (const T& pointXY, const
Vector2D2<T>& point);

template <typename T>
class Vector2D2
{
public:
     // if I commend this out the code compiles with no problems
     Vector2D2 operator * (const Vector2D2& point) const { return
     *this; }

     friend Vector2D2<T> operator* <> (const T& pointXY, const
Vector2D2<T>& point);

     int y;
private:
     T b;
};

template <typename T>
Vector2D2<T> operator* (const T& pointXY, const Vector2D2<T>& point)
{
     Vector2D2<T> cc;
     cc.b = point.b * pointXY;
     return cc;
}

int main()
{
     Vector2D2<double> jj, mm;
     mm = 3.0 * jj;
     std::cout<<mm.y;
     return 0;
}

As I said before, you need to tell the computer the friend operator* is
not from the closest scope (class Vector2D2), but from another scope.
Unfortunately, just adding "::" for global scope does not work as this
gets merged to the preceding token (Vector2D2<T>). One solution might be
to put the operator* in a namespace (all custom C++ code ought to be in a
namespace anyway, so this should not a problem). The following at least
compiles with g++ (but crashes the MSVC++ compiler ;-): Probably there
are other cleaner solutions...

#include <iostream>

using namespace std;
namespace abc {

  template <typename T> class Vector2D2;
  template <typename T> Vector2D2<T> operator * (const T& pointXY, const
Vector2D2<T>& point);

  template <typename T>
  class Vector2D2
  {
  public:
    // if I commend this out the code compiles with no problems
    Vector2D2 operator * (const Vector2D2& point) const { return *this; }

    friend Vector2D2<T> abc::operator*<>(const T& pointXY, const
Vector2D2<T>& point);

    int y;
  private:
    T b;
  };

  template <typename T>
  Vector2D2<T> operator* (const T& pointXY, const Vector2D2<T>& point)
  {
    Vector2D2<T> cc;
    cc.b = point.b * pointXY;
    return cc;
  }
} // namespace abc

using namespace abc;

int main()
{
     Vector2D2<double> jj, mm;
     mm = 3.0 * jj;
     std::cout<<mm.y;
     return 0;
}