ios - "let _ = ..."(让下划线等于)在 Swift 中有什么用吗？

Question

使用 let _ = ... 有任何目的吗？

我看过关于 What's the _ underscore representative of in Swift References? 的问题和答案我知道下划线 can be used to represent a variable that isn't needed .

如果我只需要元组的一个值，如上面链接中的示例所示，这将是有意义的:

let (result, _) = someFunctionThatReturnsATuple()

不过，我最近遇到了this code :

do {
    let _ = try DB.run( table.create(ifNotExists: true) {t in
        t.column(teamId, primaryKey: true)
        t.column(city)
        t.column(nickName)
        t.column(abbreviation)
        })

} catch _ {
    // Error throw if table already exists
}

如果我只是删除 let _ =，我不会收到任何编译器警告或错误。在我看来，这样更简单、更易读。

try DB.run( table.create(ifNotExists: true) {t in
    t.column(teamId, primaryKey: true)
    t.column(city)
    t.column(nickName)
    t.column(abbreviation)
    })

代码作者写了a book and a blog about Swift .我知道作者并非万无一失，但这让我想知道我是否遗漏了什么。

Answer 1

如果该方法被 developer documentation 中的 warn_unused_result 标记，您将收到编译器警告:

Apply this attribute to a method or function declaration to have the compiler emit a warning when the method or function is called without using its result.

You can use this attribute to provide a warning message about incorrect usage of a nonmutating method that has a mutating counterpart.