Re: Proposal for automatic generation of c++ header files

[Axel Freyn <axel-freyn at gmx dot de>]

On Fri, Jan 14, 2011 at 05:17:12PM +0200, Achilleas Margaritis wrote:
> On Fri, Jan 14, 2011 at 4:58 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> > On 14 January 2011 13:26, Achilleas Margaritis wrote:
> >> My proposal does not change the language in any way, it only is a
> >> copy-and-paste job.
> >
> > That's not true, your example with an inline member function
> > demonstrates that the compiler must be changed to support your
> > proposal, so that functions defined inline are given a non-inline
> > definition that can be linked to by other translation units.
> >
> > If it was a copy'n'paste job, it could be done by an external tool,
> > which you've disputed.
> 
> My proposal doesn't change anything of the language from the
> programmer's perspective.
I think the point Jonathan wanted to make is the following: your
original example does not compile -- and can not compile without a
change of the language. If you create by hand the auto-include file you
proposed:

main.cpp:
#include "foo.hpp"
int main() {
  Foo *foo = new Foo;
  foo->bar();
  foos.push_back(foo);
}

foo.cpp:
#include <list>
class Foo {
  public:
    void bar() {}
};
std::list<Foo *> foos;
static int data = 0;

foo.hpp:
#ifndef FOO_HPP
#define FOO_HPP
#include <list>
class Foo {
  public:
    void bar();
};
extern std::list<Foo *> foos;
#endif //FOO_HPP

There is a violation of the C++-standard:
 - when main.cpp is compiled, the compiler reads "foo.hpp" -- and it
   learns that the class "Foo" has a member-function "bar" which can be
   call from within main.cpp -- so main.cpp is compiled without problem.
 - when "foo.cpp" is compiled, the compiler does not read "foo.hpp" and
   thus it sees that the function "bar" is defined INLINE. So the
   compiler will NOT export Foo::bar() -- it will be impossible to
   link to Foo::bar() from another compilation unit.
 - finally, you try to link both compilation units. Result: main() tries
   to call a function "Foo::bar()" -- but that does not exist ==>
   compilation error.
On my machine, I get exactly that:
g++ main.cpp foo.cpp
/tmp/ccszFpug.o: In function `main':
main.cpp:(.text+0x27): undefined reference to `Foo::bar()'
collect2: ld returned 1 exit status

So in fact you have to change the language, I believe (at least it is
necessary to remove the rule "if a member function is defined inline in
a class, it is marked as "inline" and NOT exported"). But may be there
are also other problems?

HTH,

Axel