变量、对象和引用之间有什么区别? [英] What is the difference between a variable, object, and reference?

问题描述

变量、对象和引用之间究竟有什么区别?

Exactly what are the differences between variables, objects, and references?

例如:它们都指向某种类型，并且它们都必须持有值(当然除非你有临时的可为空的类型)，但它们的功能和实现究竟有何不同?

For example: they all point to some type, and they must all hold values (unless of course you have the temporary null-able type), but precisely how are their functions and implementations different from each other?

示例:

Dog myDog = new Dog(); //variable myDog that holds a reference to object Dog
int x = 12; //variable x that hold a value of 12

它们有相同的概念，但它们有何不同?

They have the same concepts, but how are they different?

推荐答案

(为了清楚起见，我在这里给出的解释特定于 Java 和 C#.不要假设它适用于其他语言，尽管有些可能.)

(Just to be clear, the explanation I'm giving here is specific to Java and C#. Don't assume it applies to other languages, although bits of it may.)

我喜欢用一个比喻来告诉别人我住在哪里.我可能会在一张纸上写下我的地址:

I like to use an analogy of telling someone where I live. I might write my address on a piece of paper:

• 变量就像一张纸.它拥有一个价值，但它本身并不是价值.你可以划掉那里的任何东西，改写其他东西.
• 我写在纸上的地址就像一个参考.这不是我的房子，但它是一种导航到我家的方式.
• 我的房子本身就像一个物体.我可以给出对同一个对象的多个引用，但只有一个对象.

这有帮助吗?

值类型和引用类型之间的区别是写在纸上的.例如，这里:

The difference between a value type and a reference type is what gets written on the piece of paper. For example, here:

int x = 12;

就像一张纸，上面直接写着数字12.鉴于:

is like having a piece of paper with the number 12 written on it directly. Whereas:

Dog myDog = new Dog();

不会将 Dog 对象本身的内容写在纸上 - 它会创建一个新的 Dog，然后在该纸上写入对狗的引用.

doesn't write the Dog object contents itself on the piece of paper - it creates a new Dog, and then writes a reference to the dog on that paper.

在非类比方面:

• 变量表示内存中的存储位置.它有一个名称，您可以在编译时通过它来引用它，并且在执行时它有一个值，该值将始终与其编译时类型兼容.(例如，如果您有一个 Button 变量，该值将始终是对 Button 类型的对象或某个子类的引用 - 或 null 参考.)
• 对象是一种独立的实体.重要的是，变量或任何表达式的值永远是一个对象，只是一个引用.一个对象有效地包括:
  • 字段(状态)
  • 类型引用(在对象的生命周期内永远不会改变)
  • 监视器(用于同步)
  • A variable represents a storage location in memory. It has a name by which you can refer to it at compile time, and at execution time it has a value, which will always be compatible with its compile-time type. (For example, if you've got a Button variable, the value will always be a reference to an object of type Button or some subclass - or the null reference.)
  • An object is a sort of separate entity. Importantly, the value of a variable or any expression is never an object, only a reference. An object effectively consists of:
    • Fields (the state)
    • A type reference (can never change through the lifetime of the object)
    • A monitor (for synchronization)

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