Using delegated properties in data classes

Language Design
1

Currently it’s not possible to use delegated properties in the primary constructor of data classes or any class for that matter.

e.g.

data class User(val name: String, var password: String by PasswordChecker())

Has this possibility been considered? It would be great if one can still benefit from the automatically derived features (equals/hashcode/tostring/copy/…) without sacrificing the power of delegation.

2 Likes
2

I don’t really understand how this would work. I guess PasswordChecker supplies a getter and a setter. And the constructor should than call the setter of the PasswordChecker? But what if the delegate only supplies a getter. How is the value to be set than?
Pls correct me if I’m wrong.

3

The compiler should raise an error in the case of a read-only property delegate, just like when you would apply a read-only property in the currently supported way:

data class User(val name: String) {
  var password: String by PasswordChecker()
}
1 Like
4

Here is an example that ends up with more boilerplate than what is needed.

class Commit(
    id: EntityID<Int>,
) : IntEntity(id), ICommit {
    companion object : IntEntityClass<AbstractRedCommit>(RedCommits)

    var sha by Commits.sha
    var commitTime by Commits.commitTime
    val icommit = object : CommitObject() {
        override fun getCommitSha(): String {
            return sha
        }

        override fun getCommitTime(): LocalDateTime {
            return commitTime!!
        }
    }

    /*
    * Ideally this is automatically delegated to icommit automatically. I can't figure out a way to do that.
    * */
    override fun getCommitSha(): String {
        return icommit.getCommitSha()
    }

    override fun getCommitTime(): LocalDateTime {
        return icommit.getCommitTime()
    }

    override fun isCommitOlder(commit: ICommit): Boolean {
        return icommit.isCommitOlder(commit)
    }
}