2.1 and 2.1-8

Problem: template declaration in one source file (.h), template member function
definitions in another source file (unit A), and template member function use in
a third source file (unit B). Linker complains that symbol for template member
function is undefined.

// test.h
template <class T>
struct A {
void fn();
};

// test.cpp
#include "test.h"
template <class T> void A<T>::fn() { }

// main.cpp
#include "test.h"
void main() {
A<int> a;
a.fn();
}

// Error 42: Symbol Undefined
// ?fn@?$A@H@@QAEXXZ (void syscall A<int >::fn(void ))

This has slowly become a high priority for me.

Richard

Unfortunately, you'll need to put the template bodies into the header files. That's the way it is with a separate compilation model. The good part is that there's no penalty for doing so (other than a bit slower compilation speed).

"Richard" <Richard_member@pathlink.com> wrote in message news:b0pm9c$3p1$1@digitaldaemon.com...
> 2.1 and 2.1-8
>
> Problem: template declaration in one source file (.h), template member
function
> definitions in another source file (unit A), and template member function
use in
> a third source file (unit B). Linker complains that symbol for template
member
> function is undefined.
>
> // test.h
> template <class T>
> struct A {
> void fn();
> };
>
> // test.cpp
> #include "test.h"
> template <class T> void A<T>::fn() { }
>
> // main.cpp
> #include "test.h"
> void main() {
> A<int> a;
> a.fn();
> }
>
> // Error 42: Symbol Undefined
> // ?fn@?$A@H@@QAEXXZ (void syscall A<int >::fn(void ))
>
> This has slowly become a high priority for me.
>
> Richard
>
>

In article <b0ptcq$8f3$1@digitaldaemon.com>, Walter says...
>
>Unfortunately, you'll need to put the template bodies into the header files. That's the way it is with a separate compilation model. The good part is that there's no penalty for doing so (other than a bit slower compilation speed).

Ok, that's fine for me, and a non exhaustive search of boost lib code indicates awareness of compilation models, but it does create problems with libs that are unaware.. just to be clear: are you saying that this behavior is by design, and that no change is anticipated?

Richard

"Richard Grant" <fractal@clark.net> wrote in message news:b0r5gj$t14$1@digitaldaemon.com...
> In article <b0ptcq$8f3$1@digitaldaemon.com>, Walter says...
> >Unfortunately, you'll need to put the template bodies into the header
files.
> >That's the way it is with a separate compilation model. The good part is that there's no penalty for doing so (other than a bit slower compilation speed).
> Ok, that's fine for me, and a non exhaustive search of boost lib code
indicates
> awareness of compilation models, but it does create problems with libs
that are
> unaware.. just to be clear: are you saying that this behavior is by
design, and
> that no change is anticipated?

Yes. I also don't think you'll find any code otherwise. I think all the existing compilers work that way.

In article <b0rq1h$17o7$1@digitaldaemon.com>, Walter says...

>Yes. I also don't think you'll find any code otherwise. I think all the existing compilers work that way.

In some other thread on Template definitions in different translation units, we were talking about irrelevant details. According to 14-7 of the template introduction, The export keyword handles this issue. But this does not work:

export template <class T> struct A {
// Error: missing decl-specifier-seq for declaration of 'export'
void fn();
};

template <class T> void A<T>::fn() { }

int main() {
A<int> a;
a.fn();
}

Richard

Exported templates do not work yet.

"Richard Grant" <fractal@clark.net> wrote in message news:b3ngi1$2jmr$1@digitaldaemon.com...
> In article <b0rq1h$17o7$1@digitaldaemon.com>, Walter says...
>
> >Yes. I also don't think you'll find any code otherwise. I think all the existing compilers work that way.
>
> In some other thread on Template definitions in different translation
units, we
> were talking about irrelevant details. According to 14-7 of the template introduction, The export keyword handles this issue. But this does not
work:
>
> export template <class T> struct A {
> // Error: missing decl-specifier-seq for declaration of 'export'
> void fn();
> };
>
> template <class T> void A<T>::fn() { }
>
> int main() {
> A<int> a;
> a.fn();
> }
>
> Richard
>
>
>
>
>

For template function declarations and definitions to be in different files, the declarations must be available in the file in which they are defined. That is whats the root of all problems in this case

For template class/function definitions to be in one file and their declarations in another, you can use one of 2 methods:-

In the file test.h, add #include "test.cpp" after the template struct definition

Use the export keyword in test.cpp as follows:-
export template<class T> void A<T>::fn() {}
However, most compilers dont support the keyword export. So, you
can do one of two things...
1> Follow method one above
2> Keep template function/struct/class declarations and
definitions in the same file.