Error in friend function in class templates

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#include "stdafx.h"
#include <iostream>
using namespace std;
#include <string>


template<class T>
class students
{
    friend void shows(students<T> &);

private:
    T name;
    void ShowInfo();

public:
        students(T x)
    {
        name=x;
    }
};

template<class T>
 void shows(students<T> &stu)
{
    cout<<stu.name;
}


int _tmain(int argc, _TCHAR* argv[])
{

    students <int>  mat(10);
    shows(mat);

    return 0;
}

Im getting this error:
Error 1 error LNK2019: unresolved external symbol "void __cdecl shows(class students<int> &)" (?shows@@YAXAAV?$students@H@@@Z) referenced in function _wmain

Posted 30-Jan-13 5:19am

missak boyajian

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Matthew Faithfull · Answer 1 · 2013-01-30T05:42:00

I don't think the friend-ness is the issue here although template friends are buggy to some extent in all versions of Visual Studio before 2012 as far as I can tell.
The problem seems to be that the compiler is 'probably' not generating code for the shows template function because it doesn't think it's being called instead it's looking for a function with a different signature which has not been written, trying to import it from the available libraries and failing.
I would try removing the friend declaration and doing something like this:

template< class T >
void shows( T& stu )
{
cout << stu.name;
}

int _tmain( int arg, _TCHAR* argv[] )
{
students< int > mat(10);
shows< students >( mat );

return 0;
}

This should then give you the error saying you can't access a private member of students. Then you can put the friend declaration back in based on the function signature it shows you in the error message.

templates are great for stretching the brain. good luck.

sja63 · Answer 2 · 2013-01-30T23:31:00

Solution 2

An interesting question: C++ and friendship...

Here is the complete code:

C++

#include "stdafx.h"
#include <iostream>
using namespace std;

template<class T>
class students
{
public:
    friend void shows(T&);
    T name;
    void ShowInfo();
public:
    students(T x)
    {
        name=x;
    }
};

template<class T>
void shows(students<T> &stu)
{
    cout << stu.name << endl;
}

int _tmain(int argc, _TCHAR* argv[])
{

    students<int> mat(10);
    shows(mat);

    return 0;
}

It was also the privateness which made problems.

Best regards

Posted 30-Jan-13 23:31pm

sja63

Updated 30-Jan-13 23:37pm

v2

Comments

Matthew Faithfull 31-Jan-13 5:47am

If everything in students< int > is public then you do not need the friend declaration. friend is meant to give specific classes or functions access to private functions and data. The tricky thing is here is specifying your friend declaration correctly. void shows(T&) is not the same function as template< class T > void shows( students<T>& ).

What compilation error message do you get exactly if you comment out the friend line and make T name a private member?

sja63 31-Jan-13 6:08am

1> friend.cpp
1>c:\friend\friend.cpp(26): error C2248: "students<t>::name": Kein Zugriff auf private Member, dessen Deklaration in der students<t>-Klasse erfolgte.
1> with
1> [
1> T=int
1> ]
1> c:\friend\friend.cpp(14): Siehe Deklaration von 'students<t>::name'
1> with
1> [
1> T=int
1> ]
1> c:\friend\friend.cpp(33): Siehe Verweis auf die Instanziierung der gerade kompilierten Funktions-template "void shows<int>(students<t> &)".
1> with
1> [
1> T=int
1> ]
1>
1>Fehler beim Erstellen
1>
1>Verstrichene Zeit 00:00:00.74
========== Erstellen: 0 erfolgreich, Fehler bei 1, 0 aktuell, 0 übersprungen ==========

sja63 31-Jan-13 6:10am

One can replace the line
friend void shows(T&);
by
void shows(T&);

So the friend declaration is for what?

I am confused

Matthew Faithfull · Answer 3 · 2013-01-31T00:22:00

OK, that's what I thought although I don't often get error messages in German :-)

friend A

inside class S means: 'grant A access to the private members of S'

The problem is that you have said 'grant Z access to private members of S' and then tried to access private members of S from A

try changing your friend declaration to

friend template<> void shows( T& );

unfortunately this depends on your exact compiler version. In some it might have to be:

friend template void shows( T& );
friend template void shows< int >( students< int >& );

or even

friend template void shows<>( T& );

and in some it will not work however you write it. :-(

This I'm afraid is the confused state of C++ compilers when it comes to combining templates with friends.
Be gald you're not trying to export any of this on the interface of a shared library or it all gets much worse.

Error in friend function in class templates

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