5 ms·

IMO that's the typical experience with many of the features in modern C++ standards. You read about a really neat useful thing they added, something that seems

by MawKKe 2y ago

IMO that's the typical experience with many of the features in modern C++ standards. You read about a really neat useful thing they added, something that seems to provide a safe and practical way to overcome a shortcoming in the language. You may even get a little excited...until you try to actually use it and realize its full of new footguns and weird limitations

fgkhax 2y ago
Yes, you read about std::variant on a blog and think that it is a sum type. Then you try it out and realize that it's a thin (type-safe) wrapper over tagged unions that is at least three times slower and has about 5 unreadable alternatives that replace simple switch statements. Then you find out that members of a "variant" are not really variant members but just the individual types that can be assigned to a union. For example, assigning to a non-const reference does not work (and obviously cannot work once you realize that std::variant is just syntax sugar over a tagged union). Most of these new additions since C++11 are just leaky abstractions and wrappers.
- galkk 2y ago
  Ohh, and to make the use of a variant to look like pattern match over type you need to copy paste some template magic. https://schneide.blog/2018/01/11/c17-the-two-line-visitor-explained/ https://schneide.blog/2018/01/11/c17-the-two-line-visitor-ex... variants are a such disappointment at every step of trying to use them
- nly 2y ago
  A few code snippets of what you see as weaknesses of std::variant may be appropriate, as I couldn't figure out your complaint. Assigning to a variant taken by non-const& works fine for me. I personally would have liked to see recursive variant types and multi-visitation (as supported by boost::variant).
  - amluto 2y ago
    I think the comment means: std::variant<int&, etc> does not work well.
    - nly 2y ago
      Just use std::variant<int*, ...> References in C++ are just sugary pointers.
      - robmccoll 2y ago
        References have one important property over pointers. They cannot be null.
        tstack 2y ago
        They most certainly can be null as can “this”.
        usefulcat 2y ago
        Only after dereferencing a null pointer, which would be the actual problem, not the fact that some reference or this is null.
        Matheus28 2y ago
        You cannot have a null reference unless you invoke undefined behavior first.
        cma 2y ago
        const ref lifetime extension is important to, or operator overloading wouldn't be workable.
        gpderetta 2y ago
        Reference_wrapper then.
        LowLevelMahn 2y ago
- gpderetta 2y ago
  > and obviously cannot work once you realize that std::variant is just syntax sugar over a tagged union It would be easy to make it work, there isn't necessarily a strict relation between the template parameter and the actual stored object. Not having reference variant members was a conscious decision, same as optional<T&>. Hopefully this will be fixed in the future.

by MawKKe 2y ago

fgkhax 2y ago

Yes, you read about std::variant on a blog and think that it is a sum type. Then you try it out and realize that it's a thin (type-safe) wrapper over tagged unions that is at least three times slower and has about 5 unreadable alternatives that replace simple switch statements. Then you find out that members of a "variant" are not really variant members but just the individual types that can be assigned to a union. For example, assigning to a non-const reference does not work (and obviously cannot work once you realize that std::variant is just syntax sugar over a tagged union). Most of these new additions since C++11 are just leaky abstractions and wrappers.

galkk 2y ago
Ohh, and to make the use of a variant to look like pattern match over type you need to copy paste some template magic. https://schneide.blog/2018/01/11/c17-the-two-line-visitor-explained/ https://schneide.blog/2018/01/11/c17-the-two-line-visitor-ex... variants are a such disappointment at every step of trying to use them
nly 2y ago
A few code snippets of what you see as weaknesses of std::variant may be appropriate, as I couldn't figure out your complaint. Assigning to a variant taken by non-const& works fine for me. I personally would have liked to see recursive variant types and multi-visitation (as supported by boost::variant).
- amluto 2y ago
  I think the comment means: std::variant<int&, etc> does not work well.
  - nly 2y ago
    Just use std::variant<int*, ...> References in C++ are just sugary pointers.
    - robmccoll 2y ago
      References have one important property over pointers. They cannot be null.
      - tstack 2y ago
        They most certainly can be null as can “this”.
        usefulcat 2y ago
        Only after dereferencing a null pointer, which would be the actual problem, not the fact that some reference or this is null.
        Matheus28 2y ago
        You cannot have a null reference unless you invoke undefined behavior first.
      - cma 2y ago
        const ref lifetime extension is important to, or operator overloading wouldn't be workable.
      - gpderetta 2y ago
        Reference_wrapper then.
      - LowLevelMahn 2y ago
gpderetta 2y ago
> and obviously cannot work once you realize that std::variant is just syntax sugar over a tagged union It would be easy to make it work, there isn't necessarily a strict relation between the template parameter and the actual stored object. Not having reference variant members was a conscious decision, same as optional<T&>. Hopefully this will be fixed in the future.