双向和单向关联 UML [英] bi-directional and uni-directional associations UML

问题描述

虽然我认为我了解 aggregation 和 composition，但我很难理解 bi-directional 和 uni-directional关联.我已经阅读了 bi-directional 关联，两个类都知道彼此和 uni-directional 关联只有一个类知道的关系.然而，这个解释对我来说似乎太抽象了，我会想知道这对我的代码和我正在编写的程序特别意味着什么.如果你能连同解释一起提供一个简单的例子，那就太好了这两个如何转换为代码(我更喜欢 c++，但它可以是任何东西，包括伪代码)

While I think I understand aggregation and composition, I am having difficulties understanding bi-directional and uni-directional association. I've read that with bi-directional association, both classes know about each other and with uni-directional association only one of the classes is aware of the relationship. However this explanation seems too abstract for me and I would like to know what this in particular means for my code and for the program I am writing. It would be very nice if you could, along with explanation, provide a simple example of how these two translate to code (I prefer c++, but it can be anything including pseudocode)

推荐答案

双向关联可从两端进行导航.例如，给定以下类(为简单起见，假设两端的关联为 0..1)

Bi-directional associations are navigable from both ends. For instance, given the following classes (for simplicity, suppose that the association is 0..1 in both ends)

class Parent {
  Child* children;
}

class Child {
  Parent* parent;
}

您可以从 Parent 转到其子项，反之亦然:父项知道其子项，子项知道其父项，并且(如果 this.parent!=null) this.parent.child==this(否则不会是同一个关联).

you can go from a Parent to its child, and vice-versa: the parent knows about its child, the child knows about its parent, and (if this.parent!=null) this.parent.child==this (othewise it would not be the same association).

Parent <---------> Child

但是，如果Child中没有指向Parent的指针:

However, if there were no pointer to Parent in Child:

class Child { }

这将是一个单向关联:你可以从父到子，但不能从子到父.

it would be an uni-directional association: you can go from parent to child, but you cannot go back from children to parent.

Parent ----------> Child

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