Emil,

I know that the second snippet is illegal in C++, but your statement was that const is only a part of type system and has no impact on optimization, I just answered you with my doubts, and made a first obvious example with NRVO, which highly depends on const.

const might also involve some optimizations, e.g. in multi-threaded context. Let's imagine a value being passed to a function by const reference. This function starts 2 or more threads and passes the value to them as reference to a const object as well. What happens in this context? As long it is not volatile it might be cached in a register, but I think that if the value is const, compiler has stronger assumption, that this value is good enough to be cached for all threads...
 

On Fri, Mar 20, 2009 at 1:56 AM, Emil Dotchevski <emildotchevski@gmail.com> wrote:
On Thu, Mar 19, 2009 at 4:29 PM, Ovanes Markarian
<om_boost@keywallet.com> wrote:
> What about RVO and NRVO?
>
> You still can bind a temporary to a const reference. This is a simple
> example on how it is possible to do optimizations. Or am I mistaken?
>
> Example:
>
> std::string foo();
>
>
> void bar()
> {
>    std::string const& bound_temporary = foo();
>
>    std::cout << bound_temporary;
> }
>
> whereas this is invalid:
>
> void invalid_bar()
> {
>    std::string& bound_temporary = foo(); //will not work, but AFAIK some
> compilers support it
> }

Right, your first snippet is valid and the second is ill-formed.

> So compilers can do optimizations based on constness.

What optimizations do you have in mind? The second snippet is simply
illegal C++ (should not compile.)

Emil Dotchevski
Reverge Studios, Inc.
http://www.revergestudios.com/reblog/index.php?n=ReCode
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