On Thu, Jul 9, 2009 at 9:41 AM, Stewart, Robert<Robert.Stewart@sig.com> wrote:
Gottlob Frege wrote:
Assuming direction is the exemplar Vladimir has been using all along, that won't compile because direction has no default constructor. The underlying behavior is the same as that of lexical_cast:
direction result; std::istringstream stream(str); stream >> result; return result;
That requires a default constructor.
This code:
direction d(convert<direction>::from(str, direction::up));
avoids the need for a default constructor via code like the following:
direction result(direction::up); std::istringstream stream(str); stream >> result; return result;
Maybe the default/error form: convert<direction>::from(str, down) shouldn't compile either (ie don't use error return as init value).
There isn't a choice. In order to extract a direction from an istringstream, there must be a direction instance. If direction has no default constructor, then the result *must* be initialized from the error return value. I don't know what you think is or could be happening, but I hope the code I've shown above clarifies the matter.
I'm sure you are right, and the real limitation comes down to how streams and << work. I was also trying to imagine the no_throw case: direction result(default_value); try { stream >> result; } catch (...) { result = error_value; } return result; Although I'm not even sure - if you can guarantee that "return result" doesn't throw (in some stupid complicated copy constructor) - if that is the meaning of no_throw - if that is the meaning of "error" (ie stream >> result could still have an 'error' but not have thrown, depending on how it works, and meaning, etc) - and still direction needed to be initialized, because, basically, it doesn't have a construct-from-stream: { return direction(stream); } nor do most types, thus we use >> instead, as is defacto. Basically I was leaving the implementation open to my imagination, and focusing more on how it looks from the outside.
_____ Rob Stewart
Tony