Re: [boost] Re: lexical_cast with default value

From: Kevlin Henney (kevlin_at_[hidden])
Date: 2005-05-12 09:05:36

In message <c36e3cc4d205f62b0ca8f74dfe7ca778_at_[hidden]>, Dave Dribin 
<dave-ml_at_[hidden]> writes
>>
>> 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.
>
>Personally, I think there is a need to to specify the default value, 
>rather than rely on the default constructed value.  For example:
>
>  int httpPort = convert_to<int>(aString, 80);
>
>Maybe that can be done in similar vain to (1) or (2):
>
>  int httpPort = lexical_cast<int>(default_value<int>(aString, 80));
>
>I don't know templates well enough to know if that's possible.  Seems 
>like the implementation would need to convert 80 into a string, and 
>then back to an int, though, as  default_value would need to return a 
>string in the above example, right?

I think that the optional approach (2) would be more appropriate if this 
proves to be a killer need. It is lighter in weight in terms of syntax 
and implementation, and is ultimately more general purpose.

However, the example above doesn't seem to be quite the killer example 
given that 0 is readily identifiable as an inappropriate port number. 
Yes, it's a convenience, but the convert_to (or perhaps convert_else) 
syntax is simpler than using either (2) or (3).

Kevlin
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