Re: [boost] Re: lexical_cast with default value

From: Kevlin Henney (kevlin_at_[hidden])
Date: 2005-05-07 03:50:04

In message <2360aa6f4e6f12532502448436b50451_at_[hidden]>, Dave Dribin 
<dave-ml_at_[hidden]> writes
>>
>> 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?

I think that as a named entity, lexical_cast is fairly well settled, so 
I am not sure that it could be replaced without a proper deprecation 
plan. Given the less frequent need for the defaulted form, I am not sure 
that such a significant is easily justified.

However, when I consider what the most common use of a defaulted value 
would likely be, based on my own experience of similar features, other 
library designs and indeed your example above, it appears that in the 
event of non-conversion the explicitly default constructed value of the 
target type is the most common use. Creating an interface for that would 
be somewhat easier.

There are essentially three practical options for doing this, as I see 
it (a couple of others come to mind, but don't qualify as being in line 
with the existing design):

(1) Provide a wrapper type for the source on which lexical_cast is 
overloaded and will handle non-conversion separately, eg 
lexical_cast<T>(defaulted(a)) or lexical_cast<T>(nonthrowing(a)).

(2) Specify a target type that indicates a non-throwing outcome. The 
following suggestion has come to me from a colleague of Thomas Witt's: 
lexical_cast< optional<T> >(a).

(3) Define another cast-like function that is specifically named, eg 
nothrow_lexical_cast<T>(a).

In terms of simplicity and directness my preference is for (3), although 
I am not wed to the name. I would be interested to hear any thoughts on 
this.

Kevlin
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