Usage of double colon:: in kotlin

In Kotlin, ::is an operator that can be used to refer to functions, properties, or constructors. The specific usage is as follows:

1. Reference functions: You can use ::operators to reference a function. For example, ::printlnit can be used to reference printlnfunctions.

   fun printWithPrefix(prefix: String, message: String) {
       println("$prefix $message")
   }
   
   fun main() {
       val prefix = "Message:"
       val message = "Hello, Kotlin!"
       val printer = ::println  // 将println函数引用赋值给printer变量
       printWithPrefix(prefix, message)  // 输出: Message: Hello, Kotlin!
       printer("$prefix $message")  // 与上面的输出等价
   }
   

2. Reference properties: You can use ::operators to reference a property. For example, ::lengthit can be used to reference properties Stringof a type length.

   fun printLength(str: String) {
       println(str.length)
   }
   
   fun main() {
       val myString = "Hello, Kotlin!"
       val lengthGetter = String::length  // 将String类型的length属性引用赋值给lengthGetter变量
       printLength(myString)  // 输出: 14
       lengthGetter(myString)  // 输出: 14
   }
   

3. Reference constructor: You can use ::operators to reference a constructor. For example, ::Personit can be used to reference Personthe constructor of a class named.
4. Reference to member functions or member properties: You can use ::operators to reference member functions or member properties of a class. For example, myClass::myFunctionit can be used to reference a myFunctionclass member function named.

It should be noted that ::operators can only be used to reference existing functions, properties or constructors, and cannot be used to create new functions or properties.