Allow use of std::optional for optional arguments

[jonesmz]

This is in addition to supporting boost::optional.

[sjoubert]

Any update on merging this? This would be really nice to avoid having to convert boost::optional to std::optional after parsing.

[vprus]

Does this PR even work? It refers to std::optional without ever including <optional>.

Ideally, both include of the header and this new definition should be conditional on the compiler supporting C++17. Are you willing to make such changes?

[sjoubert]

I quickly tested this and it seems to work. But yes, I might have some time to put into this. What should I check?

• Conditional include and use of std::optional
• Adding some tests I assume?
• I don't think I can replace the current branch/PR, should I create a new one?

[vprus]

Yeah, I think you need a new branch/PR. For tests, they will be appreciated, but it's up to you.

[jonesmz]

the PR was pulled straight out of my work codebase's copy of boost. Used in production for over 5 years.

Feel free to adjust the PR directly, or to use an alternative PR written by someone else.

[vprus]

Folks,
thanks for your interest and contribution; this is now merged in #132.