Re: [boost] Boost.Optional advantages

From: David Abrahams (dave_at_[hidden])
Date: 2005-07-11 14:09:21

Andrey Melnikov <melnikov_at_[hidden]> writes:

> Each of them includes some extra operations which aren't necessary. 
> Version 1 is terrible, version 2 uses an extra default construction, 
> version 3 uses an extra memory allocation.
>
> Here is a better version:
>
> void foo( std::string * bar )
> {
>     new (bar) std::string("bar");
> }
> ...
> char bar_placeholder[sizeof(std::string];
> std::string *bar = reinterpret_cast<std::string*>(bar_placeholder);

reinterpret_cast is nonportable.

> foo(bar);
> ...
> bar->~std::string();
>
> It uses just one constructor call, no extra default constructors and no 
> dynamic memory allocation. So far after reading the documentation, it 
> seems to me that boost::optional provides a nice encapsuation for this 
> bulk of code.

Or just an opportunity for premature optimization.  On a compiler
supporting RVO and other kinds of copy elision, the "terrible" version
could be just as efficient, or more so.
-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com