Re: [Boost-users] program_options and wstrings

Yang Zhang wrote:

On Sun, Nov 8, 2009 at 11:52 PM, Vladimir Prus wrote:

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Yang Zhang wrote:

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- value vs. wvalue

If your option type can be constructed from char* (either using custom validator, or operator>>), you can use value. If your option type can be constructed from wchar_t, you can use wvalue. If both, wvalue is a better since you won't loose data no matter what kind of parser is used.

Why would you ever lose data? UTF-8 and UTF-16 are both encodings of the same set of characters. Isn't that what codecvt converts between?

But char* strings are not necessary UTF-8, they are in local 8-bit encoding that might well be KOI8-R, or whatever else. So, if you have wchar_t* argv, and your final target is 'string', you have two possible transformations: wchar_t* -> string wchar_t* -> char* -> string with 'wvalue', the first conversion will be attempted, and will fail. with 'value', the second conversion will be attempted, and might work, or might lose some data.

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Given what wstring cannot be constructed from char*, you have to use wvalue for wstring.

You can't construct a vector<...> from a char* either, yet that's legal. See my confusion? :) This is why it's unclear to me what significance value vs. wvalue have - esp. since codecvt is doing conversions anyway.

Speaking of vector, you can create a custom validator to create vector from char*, or wchar_t*, or both -- and then you still have to use value/wvalue to specify which path data should travel. - Volodya