[adgasf]

C++ match expressions would make std::variant a really nice alternative.

I assume there is already a proposal out?

[monk_the_dog]

Pattern matching is on many people's radar, including the "Direction Group" (See: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p093...). This proposal was discussed at the committee meeting last month: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p137... I'd love to see it standardized in 23, but I have no idea how likely that is. (It definately won't be in c++-20, which is now closed except for tweaks).

[exitb]

For reference, Rust version shows how much simpler that could be with pattern matching - https://gist.github.com/km216/aad5c0fa11f32aa562af2370a32208...

[ddalcino]

Until C++2z rolls out with match support, you can roll your own. Nikolai Wuttke gave a lecture at MeetingCpp 2018 (https://www.youtube.com/watch?v=CELWr9roNno) that presents code that lets you do this:

    // Client visitor code
    match(
      thingy,
      [](const PurpleThingy& ) {std::cout << "purple thingy\n";},
      [](const LittleThingy& ) {std::cout << "little thingy\n";},
      [](const auto& ) {std::cout << "any other type\n";}
    );

Working example: https://coliru.stacked-crooked.com/a/dfed39bc7fcdfb01

[Chabs]

std::visit mostly allows you to do that already:

https://coliru.stacked-crooked.com/a/be5c44281eea8bc4

Then only unfortunate missing piece of the puzzle is that there's no trivial way to create a closure out of this, so it requires a bit more manual work to propagate local state to the visitor.

[ddalcino]

If you build your visitor out of lambdas, instead of a struct, you can propagate local state to the visitor easily by using lambda captures. There are good examples of this approach at https://en.cppreference.com/w/cpp/utility/variant/visit

[Chabs]

Unfortunately, you can't leverage template substitution rules with the lambda approach. And that's really necessary if you want to have actual powerful match expressions.

[pjmlp]

For those working with C++ an answer might come in C++23.

http://open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1371r0....