🚀 go-pugleaf

In article AD5A0C5196681CA0D@sleipner.nts.mh.se, lars.farm@nts.mh.se (Lars Farm) writes:
>This is a reply to a thread in c.l.c++.moderated. The moderator asked me to
>post this in com.std.c++ instead. The thread started with someone
>complaining about a warning for an unused variable where he had const float
>x = y; in a header.
>
>Experts explained that this is as it should be for const float, but not for
>const int (I know that the std does not talk about warnings). The point is:
>The experts seems to find some kind of conceptual difference between named
>constant floats and named constant ints, such that declaring const int K =
>L; in a header is considered good practice but const float X = Y; in a
>header is not and justifies a warning. I don't think there should be any
>such difference.

I was one of the people who posted articles on that thread, and I
certainly never said there was or should be any conceptual difference.
I did say the compiler is allowed to treat them differently.

AS AN OPTIMIZATION, a non-extern const does not always need to be
instantiated.  No compiler is required to make that optimization, to
document whether it makes that optimization, or even to be consistent
about such optimizations.  That is the whole point of optimizations:
no portable and standard-conforming program can tell the difference.
You have to resort to things like examining the executable image, or
performing a timing analysis, or some other technique outside the
language definition.

Why do I say this is an optimization? The standard doesn't (and
shouldn't, in my view) say explicitly that it is sometimes ok to omit
the instantiation.  Instead, it says what the results of certain
operations must be. It turns out that no valid program can detect
that a const was omitted. Any valid test the program makes will report
that the constant was instantiated.  The compiler can choose to omit
the instantiation only if you don't try to find out that it did.
(Shroedinger, anyone?)

If you are unhappy about your compiler generating unncessary code or
data, or warning about things which you think are the compiler's own
fault, you should take that up with the vendor. In this particular
case, no language rule is being violated.

---
Steve Clamage, stephen.clamage@eng.sun.com




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