Using Reflection For Parsing Command Line Arguments

I've been very excited about reflection so I built a small library to pass command line arguments

Basic example:

struct Args
{
std::string firstname;
int age;
bool active;
};

// ./program --first-name John --age 99 --active

const auto args = clap::parse<Args>(argc, argv);

assert(args.firstname == "John");
assert(args.age == 99);
assert(args.active);

More interesting example:

struct Args
{
[= clap::Description<"host to connecto to">{}]
std::string host = "localhost";

[=clap::ShortName<'p'>{}]
std::uint16t port;

[[=clap::Env<"RETRYCOUNT">{}]]
std::uint32t retrycount;

std::optional<std::string> logfile;

[[=clap::ShortName<'e'>{}]]
bool encrypted;

[[=clap::ShortName<'c'>{}]]
bool compressed;

[[=clap::ShortName<'h'>{}]]
bool hashed;
};

// ./program -p 8080 -ec

const auto args = clap::parse<Args>(argc, argv);

assert(args.host == "localhost");
assert(args.port == 8080);
assert(args.retrycount == std::stoul(std::getenv("RETRYCOUNT")));
assert(!args.logfile);
assert(args.encrypted);
assert(args.compressed);
assert(!args.hashed);

The amount of code to handle this is actually quite minimal < 500 lines.

There's a few modern goodies that make this code work:

Reflection \[[P2996](https://isocpp.org/files/papers/P2996R13.html)\]
Annotations for Reflection [P3394\]
Contracts \[[P2900](https://isocpp.org/files/papers/P2900R14.pdf)\]
constexpr exceptions [P3068\]

I guess we don't know what "idiomatic" reflection usage is like yet, I'm interested to come back to this code in a years time and see what mistakes I made!

Link to the code: https://github.com/nathan-baggs/clap

Any feedback, queries, questions are welcome!

https://redd.it/1sv4w78
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