Question

我想为Double写一个扩展名，以便它可以给Int赋值。

extension Double {
  func toPercentage() -> Int {
    var mutableSelf = self
    var twoDigits = Double(round(1000*mutableSelf)/1000)
    return Int(twoDigits) * 100
  }
}

在线var twoDigits = Double(round(1000*mutableSelf)/1000)编译器抛出红色-Cannot use mutating member on immutable value: 'self' is immutable

但是我确实将自己重新分配给mutableSelf变量。 Double是一个结构，它不是引用类型，为什么会出现错误？

Answer 1

由于扩展名为db = get_db() # Dumping to log to check if the db actually has the row cur_e = db.execute('SELECT user_id, user_email FROM cld_user') ie = cur_e.fetchall() for i in ie: print "ID: {} | Email: {}".format(i[0], i[1]) print "end of ID dump" print user_email # The above code does print out the row (even after hitting the back button) try: # This block however, fails when user submits the form after hitting the back button print "Trying" cur_ps = db.execute('SELECT user_id, user_uid FROM cld_user \ WHERE user_email = ?', [user_email]) user_id = cur_ps.fetchone()[0] user_uid = cur_ps.fetchone()[1] print "Row exists" flash(user_id, "success") except Exception: print "Exception" cur_p = db.execute('INSERT OR IGNORE INTO cld_user \ (user_uid, user_name, user_email, user_contact) \ VALUES (?, ?, ?, ?)', [user_uid, user_name, user_email, user_contact]) user_id = cur_p.lastrowid print "No row exists" flash(user_id, "success")，因此即使调用不匹配其签名，编译器也会将Double推断为round()的{{3}}方法。此行为已报告为错误：

您可以使用引用全局C库函数

Double

或更妙的是，使用extension Double { func toPercentage() -> Int { let twoDigits = Darwin.round(1000*self)/1000 return Int(twoDigits * 100) } }方法：

Double.rounded()

或者简单地

extension Double {
    func toPercentage() -> Int {
        let twoDigits = (1000*self).rounded()/1000
        return Int(twoDigits * 100)
    }
}

Answer 2

extension Double {
    func toPercentage() -> Int {
        let twoDigits = Double((1000 * self / 1000).rounded())
        return Int(twoDigits) * 100
    }
}

无法在不可变值Swift上使用变异成员

2 个答案: