Re: [Boost-users] program_options and wstrings

Subject: Re: [Boost-users] program_options and wstrings
From: Vladimir Prus (vladimir_at_[hidden])
Date: 2009-11-09 02:52:47

Yang Zhang wrote:

> I had some questions about using program_options with wstrings. I'm a
> bit confused after reading the documentation:
> 
> http://www.boost.org/doc/libs/1_40_0/doc/html/program_options/howto.html#id1396452
>
http://www.boost.org/doc/libs/1_40_0/doc/html/program_options/design.html#program_options.design.unicode
> 
> Excerpt:
> 
> """
> - Each parser should accept either char* or wchar_t*, correctly split
> the input into option names and option values and return the data.
> 
> - For each option, it should be possible to specify whether the
> conversion from string to value uses ascii or Unicode.
> 
> - The library guarantees that (1) ascii input is passed to an ascii
> value without change, (2) Unicode input is passed to a Unicode value
> without change, and (3) ascii input passed to a Unicode value, and
> Unicode input passed to an ascii value will be converted using a
> codecvt facet (which may be specified by the user).
> """
> 
> Bullet 1 tells me the system takes both char* and wchar_t*. Bullet 2
> tells me that each option can request the string it's handed to be
> first coerced as either a char* or wchar_t*. This is the only
> char/wchar_t conversion that happens in the system (codecvt).
> 
> Why, then, can't I say value<wstring> (besides wvalue<wstring>)? And
> why do I need to specify command_line_parser vs. wcommand_line_parser?
> 
> The complexity stems from the fact that there seem to be three sets of
> variables in the code:
> 

> - command_line_parser vs. wcommand_line_parser

The first is used to parse char* command line, the second to parse wchar_t*
command line. If you use the wrong one you program will not compile.

> - [w]value<string> vs. [w]value<wstring>

Well, using 'string' or 'wstring' as option type is entirely up to you.

> - 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.

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

Does this clarify things?

- Volodya