Re: Does binding to const-reference outperform copy-initialization from returned value?

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"Alex Blekhman" <tkfx.REMOVE@xxxxxxxxx> wrote in message
news:ety3rnr3JHA.4960@xxxxxxxxxxxxxxxxxxxx
"Niels Dekker" wrote:
When VC++ 2008 SP1 compiles the following test as "Release", the
compiler inlines constValue.VirtualFunc(), but it doesn't inline
constReference.VirtualFunc(). Unfortunately...

[...]

I guess there's an opportunity for further improvement of the
optimizer of Visual C++, right?

Such optimization won't happen in the foreseeable future.
Otherwise it would defy the whole purpose of virtual functions,
namely polymorphism. In order polymorphism to work in C++ a
function call must be made via polymorphic type. The only
polymorphic types are pointers and references. So, a call to
virtual method of a class via reference cannot be optimized
without breaking C++ rules.

Except that, in this particular code, the reference is guaranteed to be
bound to an instance of Foo and not to an instance of its derived class,
and thus polymorphism is not in the picture. So the optimizer could, in
principle, generate a non-virtual call to VirtualFunc without changing
the observable behavior of this program. Remember - a compiled program
doesn't have to follow rules, it just has to behave as if it does.
--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925


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