Re: [eigen] proposal to use static const integer class members instead of enum values

[Marc Glisse <marc.glisse@xxxxxxxx>]

On Sat, 13 Dec 2014, Benoit Jacob wrote:

> 2014-12-13 11:14 GMT-08:00 Marc Glisse <marc.glisse@xxxxxxxx>:
> > On Sat, 13 Dec 2014, Benoit Jacob wrote:
> >
> >  2014-12-13 10:47 GMT-08:00 Marc Glisse <marc.glisse@xxxxxxxx>:
> > > > On Sat, 13 Dec 2014, Benoit Jacob wrote:
> > > >
> > > >  Eigen does this for storing integer constants on a class:
> > > >
> > > > > class C {
> > > > >  enum { SomeConstant = SomeValue };
> > > > > };
> > > > >
> > > > > Most other C++ code (including libstdc++ and libc++) do this instead:
> > > > >
> > > > > class C {
> > > > >  static const some_integer_type SomeConstant = SomeValue;
> > > > > };
> > > > You are missing the extra:
> > > >
> > > > const some_integer_type C::SomeConstant;
> > > >
> > > > Otherwise, passing this value by reference may yield undefined
> > > > references,
> > > > especially at -O0.
> > > That would be the case for a static-storage non-constant integer, but not
> > > for a static-storage constant integer, see:
> > > http://stackoverflow.com/questions/9219898/why-can-you-
> > > initialize-a-static-const-variable-inline-but-not-a-plain-static-c
> >
> > You misread my message. I am not complaining about the initialization but
> > about the lack of definition.
> >
> > $ cat r.cc
> > struct A {
> >   static const int i = 42;
> > };
> > void f(int const&) { }
> > int main() {
> >   f(A::i);
> > }
> > $ g++ r.cc
> > /tmp/ccqlsWDU.o: In function `main':
> > r.cc:(.text+0xf): undefined reference to `A::i'
> > collect2: error: ld returned 1 exit status
>
> Eigen is all headers, so I'm only proposing making this change in header
> files!

I don't see how that's relevant. You can rename A as Matrix and i as 
dimension if you want, and move it to a header, it will not change 
anything.

> Given that I don't see a problem with static const int it = 42; ?

Sorry, I don't think I can make my argument clearer than the above 
example, or maybe:
https://gcc.gnu.org/wiki/VerboseDiagnostics#missing_static_const_definition

Hopefully someone else on the list can tell us in what way we are 
misunderstanding each other.

--
Marc Glisse