[Bug c++/31904] fail to link to static const double

["fang at csl dot cornell dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #5 from fang at csl dot cornell dot edu  2007-05-12 16:53 -------
Subject: Re:  fail to link to static const double

> ------- Comment #3 from dennis0yang at gmail dot com  2007-05-12 09:29 -------
> Subject: Re:  fail to link to static const double
>
> I understand that the standard only specifies that static const for
> integral type can be assigned within the class. My point is that if gcc
> decides to allow the extension of assigning static const double within
> the class (good extension, I don't see why this is not in the
> standard.), then it should allow me to link to it. Note the code works
> if I change "y=-x" to "y= -1 * x", so it is clearly a bug.

To *link* to something correctly with a reference, you need EXACTLY ONE
definition, same as in C.  An in-class declaration is not a definition.
It cannot be treated as such because you'd end up with a definition in
each translation unit that includes the header (multiply defined).  The
cases in which the compile/link 'worked' for you were cases in which the
compiler substituted the value for the reference (you got lucky).  I don't
know the heuristics for the decision, maybe ask someone familiar with the
constant-folding opt.

> I also tried your suggestion before submitting the bug report. It does
> work. But I really do not want to create a .cpp file just to put that
> one line in it, much prefer to leave everything in the .h file.

Defining it once in a .cpp file is the correct thing to do, even if it is
inconvenient.

Hope this helps.

> fang at csl dot cornell dot edu wrote:
> > ------- Comment #1 from fang at csl dot cornell dot edu  2007-05-12 09:06 -------
> > You need to define Base::x out of class in some translation unit, the in-class
> > declaration alone isn't enough, though sometimes the compiler will elide the
> > reference to it if its value is known (optimization).
> >
> > const double Base::x = 1.0;


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31904