Need help on a template question

[jianqi.wang]

Hi, everyone, I started learning C++ two weeks ago. I am trying to
play with template with constant expressions for template. So I wrote
a very simple program as follows... There are three files: gfield.h,
gfield.cpp and main.c.
//////////////////////////////////////
// gfield.h

#pragma once
#include <iostream>
using namespace std;

template <int n>
class gfield
{
public:
explicit gfield(int x):_num(x%n){}
~gfield(void);
int get_value(void) const {return _num;}
private:

int _num;
};

template <int n>
ostream& operator <<(ostream& os,const gfield<n>& gf);

///////////////////////////
//gfield.cpp

#include "gfield.h"
#include <iostream>

using namespace std;

template <int n>
gfield<n>::~gfi eld(void)
{

}

template <int n>
ostream& operator <<(ostream& os,const gfield<n>& gf)
{
os<<gf.get_valu e()<<endl;
return os;
}
///////////////////////
// main.cpp

#include <cstdlib>
#include <string>
#include <cstdio>
#include <iostream>
#include "gfield.h"

using namespace std;
main()
{
gfield<2gf(3);
cout<<gf;

cout<<test_retu rn();
}

The problem I met is that there will be link error : unresolved
external symbol... Basically, the compiler could not find operator <<
and gfield<n>::~gfi eld(void). However, if I move the definition of
these two function definitions to either main.cpp or gfield.h, it
would work.

Can anyone help me on this? Thank you very much!

[jianqi.wang]

On Jul 7, 7:05 pm, "jianqi.w...@gm ail.com" <jianqi.w...@gm ail.com>
wrote:

Hi, everyone, I started learning C++ two weeks ago. I am trying to
play with template with constant expressions for template. So I wrote
a very simple program as follows... There are three files: gfield.h,
gfield.cpp and main.c.
//////////////////////////////////////
// gfield.h

#pragma once
#include <iostream>
using namespace std;

template <int n>
class gfield
{
public:
explicit gfield(int x):_num(x%n){}
~gfield(void);
int get_value(void) const {return _num;}
private:

int _num;

};

template <int n>
ostream& operator <<(ostream& os,const gfield<n>& gf);

///////////////////////////
//gfield.cpp

#include "gfield.h"
#include <iostream>

using namespace std;

template <int n>
gfield<n>::~gfi eld(void)
{

}

template <int n>
ostream& operator <<(ostream& os,const gfield<n>& gf)
{
os<<gf.get_valu e()<<endl;
return os;

}

///////////////////////
// main.cpp

#include <cstdlib>
#include <string>
#include <cstdio>
#include <iostream>
#include "gfield.h"

using namespace std;

main()
{
gfield<2gf(3);
cout<<gf;

cout<<test_retu rn();

}

The problem I met is that there will be link error : unresolved
external symbol... Basically, the compiler could not find operator <<
and gfield<n>::~gfi eld(void). However, if I move the definition of
these two function definitions to either main.cpp or gfield.h, it
would work.

Can anyone help me on this? Thank you very much!

Please ignore that cout<<test_retu rn(), I was just doing test....
Thank you.

[=?ISO-8859-1?Q?Erik_Wikstr=F6m?=]

On 2007-07-08 01:05, ji*********@gma il.com wrote:

Hi, everyone, I started learning C++ two weeks ago. I am trying to
play with template with constant expressions for template. So I wrote
a very simple program as follows... There are three files: gfield.h,
gfield.cpp and main.c.
//////////////////////////////////////
// gfield.h

#pragma once

You should use proper include guards to make the code portable.

#include <iostream>
using namespace std;

Don't use this in a header-file, it can cause unexpected behaviour and
hard to find errors.

template <int n>
class gfield
{
public:
explicit gfield(int x):_num(x%n){}
~gfield(void);
int get_value(void) const {return _num;}
private:

int _num;
};

template <int n>
ostream& operator <<(ostream& os,const gfield<n>& gf);

///////////////////////////
//gfield.cpp

#include "gfield.h"
#include <iostream>

using namespace std;

template <int n>
gfield<n>::~gfi eld(void)
{

}

template <int n>
ostream& operator <<(ostream& os,const gfield<n>& gf)
{
os<<gf.get_valu e()<<endl;
return os;
}
///////////////////////
// main.cpp

#include <cstdlib>

Don't need this one.

#include <string>
#include <cstdio>

Or this.

#include <iostream>
#include "gfield.h"

using namespace std;
main()
{
gfield<2gf(3);
cout<<gf;

cout<<test_retu rn();

Didn't include this function in the code. Try to always post compilable
(and if possible working) code.

}

The problem I met is that there will be link error : unresolved
external symbol... Basically, the compiler could not find operator <<
and gfield<n>::~gfi eld(void). However, if I move the definition of
these two function definitions to either main.cpp or gfield.h, it
would work.

Can anyone help me on this? Thank you very much!

Yes, this is a common problem with templates, in theory you should be
able to use the export keyword to solve this, but few compiler support
it. The solution is to put all the code in the header file. See also:
http://www.parashift.com/c++-faq-lit...html#faq-35.12

--
Erik Wikström

[jianqi.wang]

On Jul 7, 7:21 pm, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2007-07-08 01:05, jianqi.w...@gma il.com wrote:

Hi, everyone, I started learning C++ two weeks ago. I am trying to
play with template with constant expressions for template. So I wrote
a very simple program as follows... There are three files: gfield.h,
gfield.cpp and main.c.
//////////////////////////////////////
// gfield.h

#pragma once

You should use proper include guards to make the code portable.

#include <iostream>
using namespace std;

Don't use this in a header-file, it can cause unexpected behaviour and
hard to find errors.

template <int n>
class gfield
{
public:
explicit gfield(int x):_num(x%n){}
~gfield(void);
int get_value(void) const {return _num;}
private:

int _num;
};

template <int n>
ostream& operator <<(ostream& os,const gfield<n>& gf);

///////////////////////////
//gfield.cpp

#include "gfield.h"
#include <iostream>

using namespace std;

template <int n>
gfield<n>::~gfi eld(void)
{

}

template <int n>
ostream& operator <<(ostream& os,const gfield<n>& gf)
{
os<<gf.get_valu e()<<endl;
return os;
}

///////////////////////
// main.cpp

#include <cstdlib>

Don't need this one.

#include <string>
#include <cstdio>

Or this.

#include <iostream>
#include "gfield.h"

using namespace std;

main()
{
gfield<2gf(3);
cout<<gf;

cout<<test_retu rn();

Didn't include this function in the code. Try to always post compilable
(and if possible working) code.

}

The problem I met is that there will be link error : unresolved
external symbol... Basically, the compiler could not find operator <<
and gfield<n>::~gfi eld(void). However, if I move the definition of
these two function definitions to either main.cpp or gfield.h, it
would work.

Can anyone help me on this? Thank you very much!

Yes, this is a common problem with templates, in theory you should be
able to use the export keyword to solve this, but few compiler support
it. The solution is to put all the code in the header file. See also:http://www.parashift.com/c++-faq-lit...html#faq-35.12

--
Erik Wikström

Erik, thank you very much! It really helps.