Very simple template question

[Winston Smith]

Hi,

I have question about compiling multiple source files with templates
using gcc. It's probably obvious but I'm new to C++. I've tried to
isolate my problem and it came down to this.

Lets say my source files are :

=== foo.h ===

#ifndef __FOO__
#define __FOO__

template <class T>
T foo(T t);

#endif

=== foo.cc ===

#include "foo.h"

template <class T>
T foo(T t) {
return t;
}

=== main.cc ===

#include <iostream>

#include "foo.h"

using namespace std;

int main(void) {
cout << foo(0) << endl;
return 0;
}

=============

When I try to compile them with the following command I get :

$ g++ foo.cc main.cc
/tmp/ccrGrumE.o(.text+0x21): In function `main':
: undefined reference to `int foo<int>(int)'
collect2: ld returned 1 exit status


Does anyone see why ld does not seem to find the implementation of the
foo function in foo.cc?

Thanks in advance for your help.

Winston

 

[John Harrison]

Hi,

I have question about compiling multiple source files with templates
using gcc. It's probably obvious but I'm new to C++. I've tried to
isolate my problem and it came down to this.

Lets say my source files are :

=== foo.h ===

#ifndef __FOO__
#define __FOO__

template <class T>
T foo(T t);

#endif

=== foo.cc ===

#include "foo.h"

template <class T>
T foo(T t) {
return t;
}

=== main.cc ===

#include <iostream>

#include "foo.h"

using namespace std;

int main(void) {
cout << foo(0) << endl;
return 0;
}

=============

When I try to compile them with the following command I get :

$ g++ foo.cc main.cc
/tmp/ccrGrumE.o(.text+0x21): In function `main':
: undefined reference to `int foo<int>(int)'
collect2: ld returned 1 exit status


Does anyone see why ld does not seem to find the implementation of the
foo function in foo.cc?

Yes it's very simple, template definitions should go in header files.
Throw away foo.cc, put all the code in foo.h.

john

 

[Daniel Mitchell]

Hi,

I have question about compiling multiple source files with templates
using gcc. It's probably obvious but I'm new to C++. I've tried to
isolate my problem and it came down to this.

Lets say my source files are :

=== foo.h ===

#ifndef __FOO__
#define __FOO__

template <class T>
T foo(T t);

#endif

=== foo.cc ===

#include "foo.h"

template <class T>
T foo(T t) {
return t;
}

=== main.cc ===

#include <iostream>

#include "foo.h"

using namespace std;

int main(void) {
cout << foo(0) << endl;
return 0;
}

=============

When I try to compile them with the following command I get :

$ g++ foo.cc main.cc
/tmp/ccrGrumE.o(.text+0x21): In function `main':
: undefined reference to `int foo<int>(int)'
collect2: ld returned 1 exit status


Does anyone see why ld does not seem to find the implementation of the
foo function in foo.cc?

Thanks in advance for your help.

Winston

Have a look at
http://www.parashift.com/c++-faq-lite/containers-and-templates.html

Daniel

 

[Winston Smith]

Have a look at
http://www.parashift.com/c++-faq-lite/containers-and-templates.html

Daniel

Thanks, very interesting reading. Even before I posted my question, I
was suspecting that a solution would be to put everything in the .h file
but I did not understand why such a limitation was necessary. Now I see.

Winston