Re: [boost] Boost.Optional advantages

From: Andrey Melnikov (melnikov_at_[hidden])
Date: 2005-07-11 15:39:55

David Abrahams wrote:
> Andrey Melnikov <melnikov_at_[hidden]> writes:
> 
>>char bar_placeholder[sizeof(std::string];
>>std::string *bar = reinterpret_cast<std::string*>(bar_placeholder);
> 
> 
> reinterpret_cast is nonportable.

Why? I don't see any reasons why reinterpert_cast from char [] to 
std::string * could be nonportable. The only thing I see is that it 
isn't supported by some compilers.

Anyways, it was just an example. Don't judge me too strictly.
> 
>>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.  

I don't think that "don't give opportunities for premature optimization" 
can be a design goal :)

Don't think about this idea just as about an optimization trick. It 
doesn't require an object to have a default or a copy constructor. It do 
relax container requirements for stored types. It is useful *sometimes*.

And why premature? I didn't recommend to use this code everywhere 
instead of returning values from functions. Did you miss a paragraph? :) 
I explicitly stated:

...*In some
circumstances* default construction, dynamic memory and copying *can* be
computationally too expensive or not available at all...

Maybe I should have used "may" there. I definitely need to improve my 
English skills.

> On a compiler
> supporting RVO and other kinds of copy elision, the "terrible" version
> could be just as efficient, or more so.

It could of course. But SOMETIMES it doesn't help.

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.

Andrey