On 5/6/2011 5:45 AM, Vladimir Batov wrote:
Again, my typical concern is twofold -- result of conversion and the return value (converted or fallback). The following is my typical use-case (note the second -- to_be -- parameter that you seemingly missed in your example)
optional<hamlet_problem> t = optional_convert_to<hamlet_problem>(s, to_be);
if (!t) { message("Invalid input. Using fallback"); t = to_be; }
[...different post...]
// Constructed by whatever means available. Not the def. cntr direction fallback (direction::up); convert<direction>::result res = convert<direction>::from(str, fallback); direction reconstructed_from_str = res.value();
For big classes with value-semantics it might be quite an overhead. However, our classes are overwhelmingly pointer-semantics pimpl-based classes. That way we essentially apply something that probably can be called in-place re-initialization. For me it's somewhat over-engineered and we've been moving towards "class::class(xml::element const&)" for unserialization. Still, all the current features of 'convert' are needed. They will be (hopefully) used on smaller scale though. Your examples are hypocritical based on your earlier argument about there not being a meaningful default value for your non-defaultable types. We agreed there is no meaningful default for the "to_be or not_to_be" problem, so your usage here of defaulting it to to_be doesn't make sense. Especially when you proceed to use the return value that's invalid...
If a default value makes sense, then you use: hamlet_problem t = to_be; if (!try_convert_to<hamlet_problem>(s, t)) { message("Invalid input. Using fallback"); } It's briefer than your version. The same goes for your direction class. We really need more context to understand your use case and why either the optional<T> optional_convert_to (without a fallback/default provided, because the value isn't needed if it fails) or try_convert_to doesn't work for you. I know your argument about the output by reference, but that is merely an interface concern which other participants here haven't yet shared. -Matt