Subject: Re: [boost] [convert] are you mixing default_value and error_value?
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-07-09 09:41:43
Gottlob Frege wrote:
> On Thursday, July 9, 2009, Vicente Botet Escriba
> <vicente.botet_at_[hidden]> 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
stream >> result;
That requires a default constructor.
direction d(convert<direction>::from(str, direction::up));
avoids the need for a default constructor via code like the
stream >> 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
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_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP http://www.sig.com
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