Message du 09/05/11 21:07 De : "Stewart, Robert" A : "'boost@lists.boost.org'" Copie à : Objet : Re: [boost] [convert] no-throw and fallback feature
Vicente BOTET wrote:
But I will first comeback to the prototype that returned a pair Target,bool. As a pair is considered as a tuple we can write
tie(i,b) = convert_to(s);
I thought about that briefly at one point in the discussions.
the main problem of this prototype is that target must be copied and a default value is needed when the conversion fails.
The extra copy is not desirable when there are reasonable (or, at least, not unreasonable) alternatives to avoid it.
I agree.
If we see optional as a kind of conditional tuple and we imagine something like a conditional tie (let me call it 'ctie'), we could be able to write
ctie(i,b) = try_convert_to(s);
ctie will left i unchanged if b is true.
I presume that no assignment would be done to i if b is false.
That actually means writing this:
int i; bool b; ctie(i, b) = try_converting_to(s); if (!b) { i = fallback; std::cerr << "using fallback\n"; }
Compare that to this:
int i(fallback); if (!try_converting_to(s, i)) { std::cerr << "using fallback\n"; }
The optional returning version provides useful benefits like no extra copy, get_value_or(), and no instance of the target type if the conversion fails.
The fist is more on the line of functional paradigm. The second on the procedural one. I understand that the procedural one can be more efficient, but the functional one separates clearly what are the in a,d the out parameters. Are both alternatives exclusive?
We could also write,
if (ctie(i,b) = try_convert_to(s),b ) ....
If in addition the result of ctie is implicitly convertible to bool, we can have
if ((ctie(i,b) = try_convert_to(s)) ...
Interesting, but probably too much of a good thing.
I agree, as most of us are not used to use tie, which is really convenient in a lot of cases. tie allows to avoid the use of first and second field of a pair in a clear way. Once we are familiar with tie, ctie is less surprising to me. A better name for ctie could be opt_tie.
Besides, the same thing is done with the following which is simpler and less tricky:
int i; if (try_converting_to(s, i)) ...
If conversion to bool is implicit the bool parameter will be redundant and the preceding could be expressed as int i; if ((opt_tie(i) = try_convert_to(*int*)(s))) ... which is not so bad. Best, Vicente