Programming Forums > Application Development > C++

grumpy · Jan 16th, 2007, 5:04 AM

It might help if you specified what symbol is being reported undefined. Lacking that information, my guess is that you need to specify an extra library for the link phase, or that the missing symbol is in code you haven't shown (eg where you create an instance of your template class).

In addition to Dawei's comment about the type of matrix, presumably in the operator<<() you intended to output g.matrix[i][j];

You also haven't provided a main() function, or any code that creates/uses an instance of your template class. One "feature" of templates is that errors in their implementation often don't show up until you actually use them (as compilers often defer instantiating/specialising templates until it has no other option).

The Dark · Jan 16th, 2007, 8:01 AM

I tried it under Visual C 7 and this is the linker error:

Quote:

test.obj : error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Gauss<int,5> const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$Gauss@H$04@@@Z) referenced in function _main

It works if you don't have the code as a friend function (and make the matrix public). I'm not suggesting this as a solution, just pointing out that adding the friend to the class stuffs up the linking for some reason.

Here is the code I used:

(Toggle Plain Text)

#include <iostream>

using std::ostream;
using std::cout;

template <class T, int size = 3>
class Gauss
{
private :
int matrix[size][size];

public:

friend ostream& operator<< (ostream &stream, const Gauss<T, size> &g);
};

template <class T, int size>
ostream& operator<<(ostream &stream, const Gauss<T, size> &g)
{
for (int i = 0; i < size; i++)
          for (int j = 0; j < size; j++)
               stream << g.matrix[i][j];

return stream;
}
int main()
{
  Gauss<int, 5> me;

  cout << (Gauss<int, 5>)me;
}

I'd still go with having a public member function that does the output and call that from the global << operator.

grumpy · Jan 16th, 2007, 9:22 AM

OK; the code, apart from my pet peeves about the "using" directives, is actually correct, as I understand it.

I seem to recall come debates a couple of years back about something like this, where some experts claimed Microsoft compilers doing this sort of thing was a bug, but Microsoft claimed that their compiler is correct in rejecting such things because of an argument based on function overloading. Microsoft's argument struck me as spurious at the time.

Unfortunately, I can suggest no workaround other than the one suggested by Dark early on (don't have the operator function as a friend, and have it call a public member).

pegasus001 · Jan 18th, 2007, 6:30 AM

Hello again boys(and girls). Sorry for the late post but i hadn`t much time solving the operator<< earlier because of school. Well i have spent quite a long time reading about templates and found a book "templates : a tutorial and reference"(i think). Well here`s the deal i solved it an for anybody who is curious here is the code :

(Toggle Plain Text)

#include <iostream>

using std::ostream;
using std::cout;

template <class T, int size = 3>
class Gauss
{
private :
int matrix[size][size];

public:

friend ostream& operator<< (ostream &stream, const Gauss<T, size> &g)
{
{
for (int i = 0; i < size; i++)
          for (int j = 0; j < size; j++)
               stream << g.matrix[i][j];

return stream;
}

}
};

int main()
{
  Gauss<int, 5> me;

  cout << (Gauss<int, 5>)me;
}

So as you can see i defined the overloaded function in the template body.(sorry i didn`t try to inline, coz i was very tired). well for me it worked and i hope it will work for you too.

Now about:

Quote:

Originally Posted by DaWei

You should not include source files. Including amounts to a copy and paste operation by the compiler BEFORE COMPILATION. This is the path to multiple definition errors.

It is advisable that the definitions of the functions of a template class should be in the same file. If you want to do it separately than you have to include an

(Toggle Plain Text)

#include "functions.cpp"

in the end of the header file. If you don`t do so the compiler will compile your files but the linker will complain. Because when the header file is compiled it looks for the definition of the functions and if it dowsn`t find them it tells the linker to find them elsewhere but the two files being compiled separately the linker cannot find the definitions anywhere and stops the execution outputing a linker error.

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Jan 16th, 2007, 5:04 AM	#11
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,254 Rep Power: 5	It might help if you specified what symbol is being reported undefined. Lacking that information, my guess is that you need to specify an extra library for the link phase, or that the missing symbol is in code you haven't shown (eg where you create an instance of your template class). In addition to Dawei's comment about the type of matrix, presumably in the operator<<() you intended to output g.matrix[i][j]; You also haven't provided a main() function, or any code that creates/uses an instance of your template class. One "feature" of templates is that errors in their implementation often don't show up until you actually use them (as compilers often defer instantiating/specialising templates until it has no other option).

Jan 16th, 2007, 9:22 AM	#13
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,254 Rep Power: 5	OK; the code, apart from my pet peeves about the "using" directives, is actually correct, as I understand it. I seem to recall come debates a couple of years back about something like this, where some experts claimed Microsoft compilers doing this sort of thing was a bug, but Microsoft claimed that their compiler is correct in rejecting such things because of an argument based on function overloading. Microsoft's argument struck me as spurious at the time. Unfortunately, I can suggest no workaround other than the one suggested by Dark early on (don't have the operator function as a friend, and have it call a public member).

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Programming Forums > Application Development > C++

Template Problem