🚀 go-pugleaf

Regarding the eliding of copy constructors, John Max Skaller wrote:

>        Completely irrelevent. The compiler is permitted to elide
>the copy constructor anyhow. If this changes the observable behaviour
>of you program it's your own fault that your code is not deterministic.

There are a few points to note about "determinism":

1) The standard says that an implementation *may* get rid of some copy
constructors; it does not demand it. Thus the indeterminism lies in the fact
that the programmer does not know whether this has happened to their code,
unless the compiler is kind enough to say so (compilers are the strong
silent type, and don't usually tell you anything about what they're thinking)
2) Removing copy constructors is intended as an optimisation. But
optimisations should never change the meaning of a program - that is, the
program should produce the same results with and without the optimisation.
Clearly, removing "irrelevant" copy constructors is a different kind of
thing altogether. 

I think this part of the standard is a little dangerous. I accept that
removing copy constructors is a useful optimisation however (though I
haven't seen any benchmarks to show how big a speedup you get - perhaps
someone could post some to convince us all that it's worthwhile). There are
a few things that might improve the situation:

1) if an implemention removes a copy constructor, the standard should demand
that it issues a warning message (if the user desires...). Though I guess
most people will turn this warning off after a couple of hours, so maybe
that's not so smart.
2) the standard should include some specified way of switching off the
removal of copy constructors. If it's in the standard then it's
transportable, so no problems there. Perhaps some ugly #pragma or something?


Cheers,

William McIlhagga
william.mc@pi.se


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