Message du 04/05/11 17:05 De : "Barend Gehrels" A : boost@lists.boost.org Copie à : Objet : Re: [boost] [review] string convert
Hi Robert,
On 4-5-2011 16:33, Stewart, Robert wrote:
Barend Gehrels wrote:
int i = convert_cast(s, 17);
// template parameter as optional one there int i = convert_cast < int , throw_even_though_i_specified_a_failback_
(s, 17);
optional i ... (similar as other one with optional) I presume you mean the following, but why?
optional i = convert_cast < optional , throw_even_though_i_specified_a_failback_
(s, 17);
Good question - I included it because it was in the original list of Gordon, but I actually also wonder what is the specific usage of this function.
Gordon?
convert_cast(s, 17) works pretty well (maybe "throw_on_failure").
I think we need to clarify one thing. Vladimir library uses values for two purposes: * as a default value when the type is not default constructible * as a fail-back in case of the conversion fails And I think we should mix them. To cover the first case we can use as I said in another post a default_value metafunction that can be specialized for the non default constructible type. I agree that when a fail-back is given the user is not interested in knowing if the conversion succeeded or not, so in this case the return value should be T and not optional T. The question now is what function should be used, convert_cast or try_convert_cast. As the function doesn't throw I will use try_convert_cast, but as the function returns type T I will use convert_cast.
Even better:
auto r(try_convert_cast(s)); if (r) { i = r.get(); }
Notice also that the precedent, std::map::insert()'s return type, puts bool second, not first. I never remember the order and you used the reverse. That leaves too much room for confusion. The optional version is much better.
I agree completely. And indeed this usage with auto is convenient. One thing, users using it like this will not notice anymore that "auto" is (behind the screens now) an "optional", and might be surprised by the .get() call.
Let me comment a little more on the function try_convert_cast returning optional T. The function can not be used directly where the target type T was expected so we can not consider it to follow the cast pattern. Other try_ functions return just bool. If we follow this pattern the preceding code could be written as int i; if (try_convert(s,i)) { // do whatever you want with i; } If you want to preserve the convert_cast that returns a optional T, I will prefer to name it optional_convert_cast, so the user that will read it will be advertised that the result is an optional T. auto r(optional_convert_cast(s)); if (r) { i = r.get(); } Best, Vicente