Re: Help needed with strange compiler/linker behavior (boost, Green Hills)
On Feb 27, 3:40 pm, "Brian Neal" <bgn...@gmail.com> wrote:
I have a follow up (with some C++ content this time). I was
able to boil the problem down to a test case, and it doesn't
have anything to do with boost per se. I just had to mimic
what was going on in boost's date_generators.hpp file.
Consider this program with the following files:
/////////////////// boost/template.h
template<typename T>
class baseClass
{
public:
virtual ~baseClass() {}
// note that this must be pure virtual to see the linker problem:
virtual void vFunc() = 0;
protected:
T array[4];
};
Derived class cut; I doubt that you need it.
int innocentFreeFunction(int n);
///////////////////// boost/template.cpp (located in another directory,
///////////////////// but still in the -I include search path:
#include <boost/template.h>
const int lookup[3] = { -1, 0, 1 };
int innocentFreeFunction(int n)
{
if (n >= 0 && n < 3)
{
return lookup[n];
}
return -100;
}
As I said in my original answer, the implicite inclusion model
of templates was by far the most frequent model under Unix in
older days, and many Unix compilers probably still support it.
(G++ was a notable exception---it didn't support templates at
all until very late, and then adopted the Borland model which
was usual under Windows.) Hopefully, there are options to
inhibit it, but when all is said and done, it is probably not a
good idea, even today, to mix template and non-template
declarations in the same header. Regardless of what the
standard says, compiler vendors feel (understandably) obligated
to support pre-standard code.
[...]
Another useful point of information: this only occurs when virtual
functions are involved in the template class. If you remove all the
virtual functions, or if you don't make the base class vFunc() pure
virtual (give it a default empty implementation), the problem is not
seen.
That is strange.
Implicit inclusion only occurs when the compiler needs to
instantiate something. Since all of the function code is inline
(or at least in the header) in your example, the compiler
shouldn't need to do it. Perhaps the problem only occurs when
the compiler generates an out of line copy; an out of line copy
is needed for virtual functions in order to put the address in
the vtable.
The exact rules of when implicit inclusion will kick in can be
fairly complicated, and depend somewhat on the compiler
internals. What is certain is that it can kick in anytime there
is a template in the header. The result is that if header file
template.hpp (or template.hh or template.h or template.H)
contains a template, it is best to consider than any file named
template.cpp or template.cc or template.C might be implicitly
included, and take steps accordingly. If you want to present
both templates and non-template declarations to a user in the
same header, either put the templates in a separate file, with a
different name, and include it from the main header, or use a
different basename for the source file with the implementation
of the non-template code. (Thus, for example, in your example,
you might call the .cpp file innocentFreeFunction.cpp, rather
than template.cpp.
I've written up a trouble report to the vendor.
I suspect that all you'll get in return is an indication of the
options to use to inhibit implicite inclusion. Or maybe only a
recommendation to use different filenames. This behavior is
intentional, and is necessary to support existing, pre-standard
code.
But if anyone else has any insight into this behavior I'd love
to hear it.
Well, I don't know why you're getting implicit inclusion only in
this specific case, but as I said, implicit inclusion is a fact
of life in C++, and must be taken into consideration when
writing code.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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