C++ virtual function calls

John B. Plevyak writes:


> David L Moore (dlmoore@ix.netcom.com) wrote:
> : > for (...) {
> : > foo->virtual_method(...);
> : > }
> : > Are compilers smart enough to lift the virtual lookup out of the loop?
> : > Is this commonly done?


I don't know of any C++ compilers that are smart enough to perform this
optimization. The three I tried (g++ 2.7, an old Cfront 2.x, and SGI's
Delta-C++, which is based on the EDG front-end version 2.19) don't.


> Because of the low level semantics of C++ code, this optimization
> is not possible in general. Nothing prevents the programmer from
> overwriting the object at the end of foo to be any (compatible) type.


That's not correct. Although you can write code which does this, such
code is prohibited by the draft ANSI/ISO C++ standard (see section 3.8).
The sample program you listed has undefined behaviour.
The C++ committee was careful to make sure this optimization was allowed.


--
Fergus Henderson WWW: http://www.cs.mu.oz.au/~fjh
fjh@cs.mu.oz.au PGP: finger fjh@128.250.37.3
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