[cfe-users] Linking problem with implicit instantiation of constructor/destructor

[Jaroslav Zeman via cfe-users]

Hello,

I've run into wieird problem when trying to compile and link our programs with 
clang++ on linux instead of g++. It occurs, when:
 - template class member definitions are separated from the definition of the 
class
- no explicit instantiation is done
- member definitions are available only in some of the units, where the 
template is being used.
(Yeah, our code is a mess)

This is simple expamle:
// - template.h 
template< typename T >
struct Template {
  Template();
  ~Template();
};

void doSomething(Template& t);

// - template.cpp 
#include "template.h"

template< typename T >
Template< T >::Template() { }

template< typename T>
Template< T >::~Template() { }

void doSomething(Template& t) {
  Template new_t;
  t = new_t;
}

// - main.cpp 
#include "template.h"

int main(int argc_, char** argv_) {

  Template t;
  doSomething(t);

  return 0;
}
// - end of code

When compiled with clang++:
$ clang++ -o test main.cpp template.cpp 
/usr/bin/ld: /tmp/main-e2fa2c.o: in function `main':
main.cpp:(.text+0x2f): undefined reference to `Template::Template()'
/usr/bin/ld: main.cpp:(.text+0x4d): undefined reference to 
`Template::~Template()'
/usr/bin/ld: main.cpp:(.text+0x82): undefined reference to 
`Template::~Template()'

reading the object files using nm tool shows, the symbol for destructor 
instantiated in template.cpp is _ZN8TemplateIiED2Ev, but the main.o requires 
symbol _ZN8TemplateIiED1Ev. Notice the difference in one digit: D2 vs D1.

So symbol ...D1... is required, but only ...D2... is available. The D1 version 
is generated by clang only for explicit instantiation. g++ generates both D1 
and D2 for any type of instantiation.

Can this be considered a bug of clang++? Or does this behaior have some 
purpose?

Regards,
JZ


___
cfe-users mailing list
cfe-users@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users

Re: [cfe-users] Linking problem with implicit instantiation of constructor/destructor

[Jaroslav Zeman via cfe-users]

> 
> What happens if you change the order of the .cpp files, putting
> template.cpp first; is it stil unresolved?
> 
> clang++ -o test template.cpp main.cpp

The order doesn't matter.

> I don't believe this code is valid according to C++. I believe it would
> require an explicit instantiation of the ctor/dtor somewhere to make that
> code valid - though I don't have chapter and verse on the spec at hand just
> now to back that up.

I tried to read the c++ specs, but didn't find anything, that would clearly 
state, if this is correct or incorrect. But the specs are too complicated for 
me to understand.

My opinion is that it is bad to rely on implicit instantiation to happen 
somewhere and don't do the explicit one. So I've already fixed all these 
problems in our code and now I am just curious, what others think about it.

JZ.



___
cfe-users mailing list
cfe-users@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users