Re: [Boost-users] [Objective-C++] Specializing boost::lexical_cast

Subject: Re: [Boost-users] [Objective-C++] Specializing boost::lexical_cast
From: Kelvin Chung (kelvSYC_at_[hidden])
Date: 2011-10-15 23:40:28

On 2011-10-15 19:48:07 +0000, Chris Cleeland said:

> On Fri, Oct 14, 2011 at 8:37 PM, Kelvin Chung <kelvSYC_at_[hidden]> wrote:
>> I'm trying to specialize boost::lexical_cast to convert Objective-C objects
>> into various standard C++ classes (for example, leveraging
>> NSNumber::-intValue to create lexical_cast<int>(NSNumber*)).  However, as
>> Objective-C objects exist as pointers, I would imagine that they would fail
>> the requirements of needing well-defined streaming operator<<() in order to
>> use the stock boost::lexical_cast.  Any words of advice on how to do this in
>> a way that will do as little damage as possible?
> 
> I haven't looked at the impl for lexical_cast, but this comes to mind
> off the top of my head...
> 
> Perhaps consider a C++ wrapper around the ObjC ptr that knows how to
> query the ObjC object for its type and turn it into the correct C++
> type.  Then provide a partial specialization for lexical_cast<> and
> that type.

Upon another look at this, operator<<() is defined for generic pointers 
as well as const char* et al., and you'd need to explicitly cast to 
void* if you wanted to output a const char* as a void*, say.  So would 
a wrapper class be necessary at all?  Would just implementing streaming 
operators, like

std::ostream& operator<<(std::ostream& out, id p) { out << [p 
description]; return out; }

std::istream& operator>>(std::istream& in, NSString*& str) { ... }

(templates, but you get the point)

be all it takes for lexical_cast to work?  Or would it be better to 
just explicitly specialize lexical_cast for specific Objective-C 
sources and target types?  (This comes to play in stuff like the 
Objective-C class NSNumber, since conversion to and from integral types 
are well defined but an operator>>(std::basic_istream&, NSNumber*)... 
not so much)