Re: [boost] [review] string convert

Subject: Re: [boost] [review] string convert
From: Matt Chambers (matt.chambers42_at_[hidden])
Date: 2011-05-05 09:43:25

On 5/5/2011 6:48 AM, Stewart, Robert wrote:
> Your use case is the following, right?
>
> optional<int>  o(try_convert_cast<int>(str));
> int i;
> if (!o)
> {
>     std::cout<<  "using fallback\n";
>     i = fallback;
> }
> else
> {
>     i = o.get();
> }
>
> That's clearly inconvenient and 5 doesn't apply because the optional won't be set if the conversion fails.  To address such inconvenience, you offered convert<T>::result and pair<optional<T>,bool>  has been suggested.  Isn't optional redundant in the latter?

What happened to:
int i = fallback;
if (!try_convert_to<int>(str, i)) { cout << "using fallback"; }

A good postcondition is that try_convert_to does not modify i if the conversion 
fails. This achieves the same thing and is honestly more convenient and 
transparent than the convert<T>::result approach. I suppose you could argue that 
the preliminary assignment is inefficient, but I can't see a reasonable 
objection to it. If it really must be avoided, then of course for defaultable 
types it can look like yours (assigning the fallback in the if), but for 
non-defaultable types there's no benefit (a default_value<T> invocation is as 
bad as the preliminary assignment).

-Matt