Re: [boost] Re: Re: Zero Unnecessary Temporary Objects: A Revolution

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2002-10-13 12:13:56

Terje Slettebø wrote:
> 
> Is it so that RVO can't
> be applied if there may be a side effect, whereas NRVO can be applied,
> regardless? Having cout's in the copy constructor would certainly constitute
> a side effect.

Exactly. This is the real value the NRVO provides, it allows
optimization which do have observable side effects!

> It is likely that newer compilers will fare better when it comes to NRVO, as
> it gets implemented on more compilers.

GCC has it since v3.1 and it works well. The Intel has it although it's
buggy, but this should be fixed soon. I would personally be interested
to hear about the VC++ 7...

> >Yes. And it's not limited to the Intel C++, as the standard itself
> >requires compilers to behave this way. A compiler is basically allowed to
> >remove temporaries only if it can figure out that this does not have any
> >observable side effects.
> 
> However, that's not the case for NRV, as mentioned, is it? After all, I got
> a varying amount of cout printing, depending on whether or not NRV was used.

Correct.

> What I'm wondering is, could a value parameter be considered an NRV by the
> compiler, or does it have to be created explicitly in the function?

A by-value-parameter is copied at the callers side of a function, not
within the function itself. That is the reason why a parameter is not a
valid NRV and the optimization cannot be applied.

Thanks for all your testing :)

Regards, Daniel

--
Daniel Frey
aixigo AG - financial training, research and technology
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