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

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<Tindex(Test<T>& start, Test<T>& end, Test<T>& step);
};

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


int main() {

Test<intstart = Test<int>();
Test<intend = Test<int>();
Test<intstep = Test<int>();

Test<intrslt = 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

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

Áõê» wrote: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.
This is what your code should look like (I think):

#include <iostream>
using namespace std;

template<typename Tclass Test;

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

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


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

First, thank you for your informative tips, I rewrite my code snippet,
here it is:

#include <iostream>
using namespace std;

template<typename T>
class Test;

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

template<typename T>
class Test {
public:
friend Test<Tindex(T start, T end, T step);
};

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

int main() {

int start;
int end;
int step;

index(start, end, step);
}

It's the same as you think it is supposed to be, no problem with
compling and linking, but at runtime, a error is raised.

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

Áõê» wrote:What error do *you* get?

You're passing 'start', 'end', 'step' into the function _without_
_giving them any value_. The lvalue-to-rvalue conversion required
for argument passing causes the program to have *undefined behaviour*.

Try initialising all your variables. Or just call

index(1, 2, 3);

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

Forget about the runtime error, it's because I used 3 variables before
I define them, sorry about that, it's OK when I initialize them. But a
new linkage error show itself when I tried to assign the return value
of index to another Test<Tobject, here is the code snippet:

#include <iostream>
using namespace std;

template<typename T>
class Test;

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

template<typename T>
class Test {
public:
friend Test<Tindex(T start, T end, T step);

};


int main() {

int start = 1;
int end = 32;
int step = 1;

Test<intrslt = index(start, end, step);
}

The error is:

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

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

should be;


template<typename T>
class Test {
public:
template<typename T//look here
friend Test<Tindex(T start, T end, T step);

};

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

Hi folks,

Thanks for your informative tips, both of your answers can solve my
problem successfully. But I have a question coming into my mind:

If there is any difference between:

template<typename T>
class Test {
friend Test<Tindex<>(T, T, T);
// ^^^
};

and

class Test {
friend Test<Tindex<T>(T, T, T);
// ^^^
};

I know you've explained it, but how can I prove that they are indeed
different, thank you.

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

leomayleomay@gmail.com wrote:I am not sure they are different, to be honest. The <Tafter 'index'
is actually implied by the empty angle brackets.

Now, if you were to write

template<class T>
class Test {
template<class Ufriend Test<Uindex(U,U,U);
};

That would definitely be different. It means that _any_ instantiation
of the 'index' template is a friend of this instantiation of 'Test'.
The usefulness of that is questionable, but it would allow you to do

template<class STest<Sindex(S s1, S s2, S s3)
{
Test<blalblahsomeOtherTest;
// and then access 'someOtherTest's privates
}

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