Re: [boost] Boost.Optional advantages

From: Caleb Epstein (caleb.epstein_at_[hidden])
Date: 2005-07-11 16:55:54

On 7/11/05, Andrey Melnikov <melnikov_at_[hidden]> wrote:
> David Abrahams wrote:
>
> > On a compiler
> > supporting RVO and other kinds of copy elision, the "terrible" version
> > could be just as efficient, or more so.
> 
> I can't imagine how a compiler can avoid calling a non-trivial copy
> constructor like in case of std::string. Don't forget that such a copy
> operation includes memory reallocation, which is much more expensive
> than just a memcpy() operation.

Modern compilers do.  You can verify this with the attached test
program that wraps std::string with "noisy" constructors, assignment
op and destructor so you can see what is happening.

With gcc 3.3.4 and no optimizations I get the following output:
noisy_string::ctor: 0xfeffad90: forty-two
noisy_string::dtor: 0xfeffad90: forty-two

With MS VC++ 7.1 I get this output for any optimization levels I try:
noisy_string::ctor: 0012FEA0: forty-two
noisy_string::copy ctor: 0012FEC8: forty-two
noisy_string::dtor: 0012FEA0: forty-two
noisy_string::dtor: 0012FEC8: forty-two

With MS VC++ 2005 Beta 2 I get the same output as above with no
optimization, but at /O1 or /O2 I see NRVO kick in:
noisy_string::ctor: 0012FF54: forty-two
noisy_string::dtor: 0012FF54: forty-two

With Sun C++ I get this:
noisy_string::ctor: ffbee63c: forty-two
noisy_string::copy ctor: ffbee6c0: forty-two
noisy_string::dtor: ffbee63c: forty-two
noisy_string::dtor: ffbee6c0: forty-two

See what you get with your compiler.

-- 
Caleb Epstein
caleb dot epstein at gmail dot com