Re: [boost] Re: lexical_cast with default value

From: Dave Dribin (dave-ml_at_[hidden])
Date: 2005-05-05 10:29:07

On May 3, 2005, at 3:27 AM, Kevlin Henney wrote:
> In message <8dee398f92d54b9e2f27e107ee73052c_at_[hidden]>, Dave Dribin 
> <dave-ml_at_[hidden]> writes
>> On May 1, 2005, at 1:56 PM, Kevlin Henney wrote:
>>> This issue of providing a default value in case of failure was 
>>> discussed in the early days of lexical_cast, but not included. In 
>>> fact, if you go back far enough, opting for the default or zero 
>>> constructed value of the target rather than throwing an exception 
>>> was the original behaviour.
>>>
>>> However, the accommodation of an additional argument was not 
>>> included in lexical_cast in part for the simple reason that it 
>>> ceases to look like a cast and in part because once you start down 
>>> that road there are a number of additional arguments and options 
>>> that you might want that are "reasonable". It was felt that much of 
>>> this would be better off in a general conversions library that had 
>>> no pretensions to being cast-like.
>>
>> I agree that the default behavior should throw an exception.  It just 
>> seems silly to create a whole new library that could be done in an 
>> existing one with about 5 lines of code.  And I'd prefer a 
>> multi-argument cast-like function than include yet *another* 3rd 
>> party library with my application.  Anyhow, I have an idea or two as 
>> alternatives.  If you're interested, lemme know, otherwise I'll drop 
>> the whole idea. :)
>
> I'd be interested to hear your ideas.

Since it sounds like the main argument against adding a second argument 
is that it is no longer cast-like, the simplest idea is to rename 
lexical_cast so it is no longer cast-like.  I was thinking convert_to 
would be nice and readable:

   int n = convert_to<int>("abc");    // Should throw an exception... 
bad_conversion?
   int n = convert_to<int>("abc", 0); // Should return 0

   string s = convert_to<string>(5);

The main issue with renaming, obviously, is backward compatibility.  Of 
course, there's no need to remove lexical_cast right away, or at all, 
even.  Maybe make it deprecated, or at least implemented in terms of 
convert_to.

Thoughts?

-Dave