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Subject: Re: [boost] [optional] generates unnessesary code for trivial types
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2012-02-09 07:04:44

On Thu, Feb 9, 2012 at 3:20 PM, Andrzej Krzemienski <akrzemi1_at_[hidden]> wrote:
> Andrey, would you mind giving us a short example of how you want to use an
> optional reference? I am in the middle of designing the new optional
> interface for TR2, and came to the conclusion that in order to avoid
> counter-intuitive semantics for optional reference assignment, I had better
> remove it at all; that is, optional references are to be limited: they
> should provide no assignment. You could still use optional values and
> optional references in generic code but with reduced interface:
> template <typename T> // T is a ref or a value
> void use( std::tr2::optional<T> opt, T nval )
> {
>  if (opt) {
>    std::cout << *out; // fine
>   *opt = nval; // fine, assigning T's not optionals
>    opt = nval; // invalid if T is a ref
>    opt = opt;  // invalid if T is a ref
>  }
>  if (needToRebindAReference()) {
>    opt.emplace(nval); // valid - always rebinds
>  }
> };
> Would such a limited interface as I described above be enough for your
> generic usage of optional?

My code is not yet set in stone, and actually, it doesn't use optional
references yet. But it looks like it should be enough for my needs,
assuming optional<T&> is still going to support the same relation
operators as optional<T> with the same semantics. The general idea is
that I build a Boost.Phoenix expression (usually, a predicate or a
streaming expression) where some of the terminals may result in
optional references (references allow to avoid expensive copying).
When the expression is executed, it should operate on the referred
values, if present, or execute a fallback logic otherwise. In my
context, the required behavior is almost exactly what optional<T&>

I have a question though. Why prohibit opt = opt assignment? It looks
quite safe and has a fairly obvious behavior. If I have an optional
reference as a member of my class, the lack of assignment in optional
would force me to define a custom assignment operator for my class.
This seems to be an unnecessary requirement. Also, in the source code
I dealt with I often saw people writing something like opt =
optional<T>() to clear the value. This would break with references for
no apparent reason.

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