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#id1...
http://www.boost.org/doc/libs/1_40_0/doc/html/program_options/design.html#pr...
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