Gottlob Frege wrote:
On Thursday, July 9, 2009, Vicente Botet Escriba <vicente.botet@wanadoo.fr> wrote:
What does this does?
direction d = convert<direction>::from(str);
Not compile, I guess.
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;
You need to either write a specialization of convert, or maybe convert_traits. You had to specialize << anyhow.
The traits idea is inappropriate because the value to return for each invocation can change.
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. _____ Rob Stewart robert.stewart@sig.com Software Engineer, Core Software using std::disclaimer; Susquehanna International Group, LLP http://www.sig.com IMPORTANT: The information contained in this email and/or its attachments is confidential. If you are not the intended recipient, please notify the sender immediately by reply and immediately delete this message and all its attachments. Any review, use, reproduction, disclosure or dissemination of this message or any attachment by an unintended recipient is strictly prohibited. Neither this message nor any attachment is intended as or should be construed as an offer, solicitation or recommendation to buy or sell any security or other financial instrument. Neither the sender, his or her employer nor any of their respective affiliates makes any warranties as to the completeness or accuracy of any of the information contained herein or that this message or any of its attachments is free of viruses.