Friend function of a template class

[Ruben Campos]

I have a problem with a template function that is declared as a friend of a
template class. I'll first show the exact problem with source code:

// MyClass.hpp
template <typename T>
class MyClass
{
// ...
friend void MyFriendFunction (MyClass <T> * const ptrMyClass);
// ...
};

#include "MyClass.cpp"


// MyClass.cpp
// ...
template <typename T>
void
MyFriendFunction (MyClass <T> * const ptrMyClass)
{
// ...
}
// ...

// Main.cpp
#include "MyClass.hpp"

int
main (int argn, char ** argv)
{
MyClass <float> x;

MyFriendFunction(&x);

return 0;
}

Although MyClass <T> is a template, I still prefer to place the
implementation of its methods in a MyClass.cpp file, and not to show them
with the class declaration. But because MyClass <T> is a template, and so it
needs method definitions visible in its header file, I include MyClass.cpp
file inside MyClass.hpp file instead of building it (I don't compile
MyClass.cpp). I've tried before this file scheme, and it works fine.

Trying to build this, I receive an linker undefined external error for the
MyFriendFunction (...) symbol. I've tried two alternative ways:

a) Including the MyFriendFunction definition directly into MyClass.hpp,
after (outside) class declaration. This returns the same linker error.

b) Including the MyFriendFunction implementation directly into declaration,
into MyClass.hpp inside class declaration. This works fine and don't return
any error.

But I don't want function implementations inside class declaration, or
merely inside a header file. Can you help me with this? Thank you very much
in advance.

 

[Rob Williscroft]

Ruben Campos wrote in in comp.lang.c++:

I have a problem with a template function that is declared as a friend
of a template class. I'll first show the exact problem with source
code:

/* Forward Declaration's
*/

template <typename T>
class MyClass;

template <typename T>
void

// MyClass.hpp
template <typename T>
class MyClass
{
// ...
friend void MyFriendFunction (MyClass <T> * const ptrMyClass);

friend void MyFriendFunction<> (MyClass <T> * const ptrMyClass);

// ...
};

#include "MyClass.cpp"


// MyClass.cpp
// ...
template <typename T>
void
MyFriendFunction (MyClass <T> * const ptrMyClass)
{
// ...
}
// ...

// Main.cpp
#include "MyClass.hpp"

int
main (int argn, char ** argv)
{
MyClass <float> x;

MyFriendFunction(&x);

return 0;
}

Trying to build this, I receive an linker undefined external error for
the MyFriendFunction (...) symbol. I've tried two alternative ways:

Yep, you were declaring the friend function as a non-template.

a) Including the MyFriendFunction definition directly into
MyClass.hpp, after (outside) class declaration. This returns the same
linker error.

Your #include "MyClass.cpp" does exactly that.

b) Including the MyFriendFunction implementation directly into
declaration, into MyClass.hpp inside class declaration. This works
fine and don't return any error.

Yes, its the only way to give the non-template declaration a defenition.

Also your are right to want to avoid this as it often leads to problems:

template < typename U, typename V >
struct X
{
friend void f( U * ) {};
};

X< int, int > xii;
/* Ok */

X< int, short > xis;
/* Whoopse: A defenition for f( int * ) has already been given */

HTH.

Rob.

 

[Ruben Campos]

I've tried your solution. The linker undefined external error don't appear
now, and all works fine. I supposed that MyFriendFunction was a template
function due to its declaration inside a template class, but I was obviously
wrong.

Thank you very much for your help.