Question: parent parser and value semantics

[zhihaoy]

The mParentParsers member is currently unused. add_parents's semantics is only merging options. But consider the following: if there is only one parent parser, copy construct from an existing parser has the same effect. We only need a way to change the new instance's mProgramName.

Does ArgumentParser has value semantics? ArgumentParser is copy cunstructible, but a copy of ArgumentParser is not a distinct copy because shared_ptr are used without special handling. Parsing in the new parser will affect the old parser. But parent parser also has the "problem," unless this is by design.

If this is not by design, then we'd better to support value semantics in full in both copy-constructor and add_parents.