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Subject: Re: [boost] [review] string convert
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-10 08:51:30

Gordon Woodhull wrote:
> I am not sure if one unified function is possible, but I wonder
> if one way to define a "best conversion" function, would be to
> drop lexical_cast's type-to-stream-to-type default, which is
> IMHO what really messes up thinking about type-to-type and
> string-to-type/type-to-string in the same sentence.
> And so convert_cast<T>(S) would use something like Convert if S
> or T is string (with any behavior determined by arguments), and
> something like Conversion if neither is.
> This is also in line with Antony Polukhin's improvements for
> lexical_cast which are mostly not lexical IIUC. I think people
> and their metaprograms expect a conversion function that is not
> actually lexical unless S or T is string.

At first blush, I agree that, without a string, there's no "lexical" in the cast. However, a UDT might contain a string and thus rely on the stream behavior and formatting. That, could, of course, be managed with a trait that indicates whether a type requires a stream for its conversion. Specializations for string types would provide the default behavior, but authors of UDTs can specialize the trait as needed.

If the trait indicates non-stream-based conversion, is it a (compile time) error to call a conversion function with manipulators, thereby implying the desire to use a stream?

With such a trait in place, the same syntax may involve a stream or not, depending upon the types involved. This fits Vladimir's idea that there need be no naming distinction between the stream-based and non-stream-based conversion functions since the trait would indicate whether a stream is needed.

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

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