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Subject: Re: [boost] [convert] Version in the Vault ready for review.
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-04-08 16:59:03

On Monday, April 06, 2009 1:23 PM
Gottlob Frege wrote:
> Sorry if I skimmed over some of the conversation too quickly, but if
> not already mentioned, I'd like to suggest passing in the result as a
> (by ref) param when you want more status information:
> A)
> int i = convert<int>::from(str); // throws if necessary
> B)
> int i = convert<int>::from(str, 0); // default supplied so no throw
> C)
> convert<int>::result res;
> int i = convert<int>::from(str, res); // no throw - error/status
> result returned in 'res'
> int i = convert<int>::from(str, 0, res); // default + status

I've been pondering this suggestion. I like that from() always returns a value and that the converted value isn't part of the result object when using one. I also like the clarity of using overloading to select the result-producing calls. I don't care much for having to default construct the result to pass it to the call, however, but it simplifies the reasoning needed to determine whether an exception will be thrown.

> I could also see using nothrow/dothrow in there somwhere, but it seems
> redundant.

dothrow is only useful in the case when the destination type has no default constructor because your case B) doesn't throw because of the default, while A) won't work for such a type. If the result type provides a means to generate the same exception as would convert<int>::from(str), then there's no need for dothrow. That is, the caller can inspect the result object and trigger the exception on conversion failure, perhaps calling a function that does both. However, that's requires extra code, whereas including dothrow in the expression somehow, makes it part of the conversion.

nothrow isn't useful in this scheme. There can be no useful return value for a nothrow call without a default or the ability to determine that the conversion failed. Passing a default or a result object handles those cases.

I think this is a nice scheme overall.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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