Olivier Langlois's blog

In my opinion, one of the most twisted part of the C++ standard (I am sure that there is more than just one twisted aspect) it is the friend function declaration in a class when you add templates to the mix. Even compiler implementers are having a hard time to get it right. I had the following code that used to compile fine in VC++6:

template< typename T >
class X
{
    friend ostream &operator<<( ostream &, const X<T> & );
};

template< typename T >
ostream &operator( ostream &os, const X<T> &t )
{
    // Print out t
    return os;
}

but now with VC++2003, I received this error message:

unresolved external symbol ostream &operator<<( ostream &, const X<T> & )

That code snippet is non conforming to the standard and in order to make the compiler happy, I had to write:

template< typename T >
class X
{
    /*
     * The '<>' is needed to say that the friend is a
     * function template.
     */
    friend ostream &operator<< <>( ostream &,
                                   const X<T> & );
};

and for those who wonder, no I did not find the solution by myself. I had to look into my book C++ Templates. From section 8.4.1 (Friend Functions):

"An instance of a function template can be made a friend by making sure the name of the friend function is followed by angle brackets...If the name is not followed by angle brackets, there are two possibilities:

1. If the name isn't qualified (in other words, it doesn't contain a double colon), it never refers to a template instance...
2. If the name is qualified (it contains ::), the name refer to a previously declared function or function template. A matching function is preferred over a matching function template..."

So my example fits case #1 description so because of a bug into VC++6 (or maybe the standard was even not complete at the time of its release...), the compiler was erroneously accepting the friend statement as referring to a function template.

So this is the whole story. What I find sad is that the rule is not intuitive at all. There is no way I can reason it next time I am in the same situation. I will either remember the rule or I will have to check back in my C++ Template book. Herb Sutter has also written an item on that topic in his book C++ exceptional Style if you are interested in a different point of view on the topic but he pretty much comes to the conclusion that this is a tricky C++ standard aspect and that a lot of compilers have troubles accepting such statements.