[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<int>& t);

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

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

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

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

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

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

  Template<int> 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<int>::Template()'
/usr/bin/ld: main.cpp:(.text+0x4d): undefined reference to 
`Template<int>::~Template()'
/usr/bin/ld: main.cpp:(.text+0x82): undefined reference to 
`Template<int>::~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