Confusing behavior when order of long/short options are swapped

从 option_description::option_description(const char*, const value_semantic*) 和文档中规定的名称格式规则开始。使用 "h,help" 在简单测试程序中复现该行为，然后确定是否应明确接受或拒绝交换顺序的名称。完成的标准是：误用不再产生令人困惑的运行时行为，并且预期行为已由测试覆盖。

Adding an option like this:

desc.add_options()( "h,help", "Print help and exit" );

In a simple test program and then trying to use --help results in a confusing error:

$ ./prog --help
libc++abi.dylib: terminating with uncaught exception of type boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::program_options::unknown_option> >: unrecognised option '--help'

The documentation for option_description::option_description(const char*, const value_semantic*) says:

The 'name' parameter is interpreted by the following rules:
if there's no "," character in 'name', it specifies long name
otherwise, the part before "," specifies long name and the part after -- short name.

However, it is easy for new users of this library to think that swapping them is OK.

The reason for this limitation isn't clear to me -- presumably, if one is "long" and the other "short", providing them in either order should be OK, and a simple test can tell which is which. In either case, the error message on misuse is needlessly cryptic and could probably be turned into an error thrown from option_description::option_description. This seems reasonable, since using "h,help" as the option name results in bad behavior regardless: the resulting executable has options --h and -h.