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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

   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

   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_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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