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 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


> > I don't think libstdc++ uses that much. If SomeConstant is "value", it
> > tries to derive from integer_constant instead.
>
>
> True, I was forgetting that it used that. I don't remember the motivation
> for that idiom, though. Does it make the compiler's job easier? Should we
> do the same?

It avoids having to define the variable 'value' for each of them. And it 
automatically brings in any niceties like implicit conversion to that 
type, operator() in C++14, etc. But that's mostly useful for some special 
forms of traits, for Eigen it may not make as much sense.

--
Marc Glisse