Re: [boost] [convert] are you mixing default_value and error_value?

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
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_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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