Re: [Boost-users] [lexical_cast] for user type extensions.

From: Balasubramanyam, Shivakumar (sbalasub_at_[hidden])
Date: 2008-04-03 12:57:51

Hi scott,

Thanks for your input. I agree with your solution as well. 

The other questions is the code location/namespace, this would work in 2
ways,

1. create the lexical_cast<HexType> in boost namespace. So the user is
transparent as to where it is getting from

2. we have a csw namespace in our project. We could create
csw::lexical_cast<HexType>

In the C++ conference, BJarne Stroutsup mentioned that it is bad idea to
put any user code in std or boost namespace. So that rules out number 1.

So I would like to know, how folks extend boost libraries for user
types. Please let me know if I am not clear.

My motivation is that, I would like folks to still refer to boost
website for api references, but I could add support for custom types

Thanks,
Shiva

-----Original Message-----
From: boost-users-bounces_at_[hidden]
[mailto:boost-users-bounces_at_[hidden]] On Behalf Of Scott McMurray
Sent: Wednesday, April 02, 2008 5:33 PM
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] [lexical_cast] for user type extensions.

On Wed, Apr 2, 2008 at 7:18 PM, Balasubramanyam, Shivakumar
<sbalasub_at_[hidden]> wrote:
>
> So example is,
>
> typedef unsigned long hexType;
> hexType value;
> lexical_cast<HexType>("0xAABBCCDD");
>
> How do we extend the boost::lexical_cast to support casting user
defined
> types?
>

HexType isn't a UDT.  If you make it a proper class/struct, then you
can overload operator>> and lexical_cast will use that.  Add in an
implicit conversion to long and the syntax you used above might even
work directly.  Something like this:

struct HexType {
    unsigned long x;
    operator unsigned long() { return x; }
}
istream &operator>>(istream &source, HexType const &h) {
    h.x = read_with_base_prefix<unsigned long>(source);
    return source;
}

And then your code will (probably) work

typedef unsigned long hexType;
hexType value;
value = lexical_cast<HexType>("0xAABBCCDD");

HTH,
~ Scott

( P.S. but if you do it for real, use names that aren't different by
case, maybe avoid the implicit conversion, etc )
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