Re: [boost] [optional] memory use for optional refs and ptrs

Subject: Re: [boost] [optional] memory use for optional refs and ptrs
From: Rutger ter Borg (rutger_at_[hidden])
Date: 2010-10-06 03:56:33

On 2010-10-06 00:19, Fernando Cacciola wrote:
> Ok, yes, in that case there is no possible doubt about the semantics, so
> the boolean can be spared.
>
> OTOH, is such a corner case that I don't imagine myself doing it in the
> sort term.
>

Although it isn't the most important case for me, it has its own section 
in the documentation, and I will probably be using lots of these. What 
also doesn't seem right to me is that, e.g.,

struct A {
     int a;
     double b;
     double c;
     std::string d;
};

struct B {
     boost::optional< int > a;
     boost::optional< double > b;
     boost::optional< double > c;
     boost::optional< std::string > d;
};

the size of A is 32 bytes in this case, and the sizeof B is 56 bytes: 
almost doubled. So, while it may be argued that the case above is a 
corner case, too, I don't it's an acceptable penalty to anyone to pay 24 
bytes for something that can be stored in 4 bits and/or in the type itself.

What would make optional useful to me, is a kind of optional that allows 
the boolean set/get stuff to be done (and possibly stored) outside of 
the optional itself. E.g., by means free functions that can be 
overloaded (and throw in the ability of defining tags to the optional 
template-parameters to be able to define a bit# in a bitset).

Cheers,

Rutger