构造函数的名称参数 [英] By-Name-Parameters for Constructors

问题描述

来自我的其他问题有没有一种方法可以获取构造函数的名称参数？我需要一种方法来提供按需在对象内按需/惰性/按名称执行的代码块，并且该代码块必须能够访问类方法，就像该代码块是类的一部分一样

coming from my other question is there a way to get by-name-parameters for constructors working? I need a way to provide a code-block which is executed on-demand/lazy/by-name inside an object and this code-block must be able to access the class-methods as if the code-block were part of the class.

以下测试用例失败：

package test

class ByNameCons(code: => Unit) {

    def exec() = {
        println("pre-code")
        code
        println("post-code")
    }

    def meth() = println("method")

    def exec2(code2: => Unit) = {
        println("pre-code")
        code2
        println("post-code")
    }
}


object ByNameCons {

    def main(args: Array[String]): Unit = {
        val tst = new ByNameCons {
            println("foo")
            meth() // knows meth() as code is part of ByNameCons
        }
        tst.exec() // ByName fails (executed right as constructor)


        println("--------")


        tst.exec2 { // ByName works
            println("foo")
            //meth() // does not know meth() as code is NOT part of ByNameCons
        }       
    }
}

输出：

foo
method
pre-code
post-code
--------
pre-code
foo
post-code

推荐答案

这是因为在创建这样的实例时：

This is because when you're making an instance like this:

val tst = new ByNameCons {
  ...
}

..实际上，您正在创建一个匿名类，例如在Java中。
上面的代码与以下内容相同：

.. you're actually creating an anonymous class, like in java. The above code is the same as:

val tst = new ByNameCons() { ... }

..而通过名称传递的正确语法是：

.. while the correct syntax for passing by-name is:

val tst = new ByNameCons( { ... } )

构造函数的括号不能与函数的省略。

You cant omit parentheses the same way for constructors as with functions.

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