C++ “has-a”和“is-a”是什么意思？
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What do "has-a" and "is-a" mean?
提问by kovenzhang
Can anyone explain what "HAS-A" and "IS-A" means between two class.
谁能解释一下两个班级之间“HAS-A”和“IS-A”的含义。
An example would helpful.
一个例子会有所帮助。
回答by paxdiablo
In the object-oriented world, a class can besomething or it can containsomething.
在面向对象的世界中，类可以是某些东西，也可以包含某些东西。
For example, a Queueclass may bea sub-class of a LinkedListclass (since a linked list can certainly be used to implement a queue). That is an is-arelationship. Everything that you can do to the linked list, you should be able to do to the queue.
例如，一个Queue类可能是一个类的子LinkedList类（因为链表当然可以用来实现一个队列）。那是一种is-a关系。您可以对链表执行的所有操作，都应该可以对队列执行。
However, the queue class may also hold otherinformation such as the number of items in the linked list (for efficiency).
然而，队列类也可能保存其他信息，例如链表中的项目数（为了效率）。
To that end, it may also define a member variable called size. That would be a has-arelationship - the queue is not a subclass of the integer, it simply contains an integer.
为此，它还可以定义一个名为 的成员变量size。这将是一种has-a关系 - 队列不是整数的子类，它只包含一个整数。
回答by Martin York
- A car is-a vehicle
- A car has-a steering wheel
- 汽车是一辆车
- 汽车有方向盘
Eg:
例如：
class SteeringWheel
{};
class Vehicle
{
virtual void doStuff() = 0;
};
class Car: public Vehicle
{
SteeringWheel sWheel;
virtual void doStuff();
};
回答by Michael Dillon
These are two common forms of relationship between two classes.
这是两个类之间的两种常见关系形式。
The HAS-A relationship refers to a class X which has a class Y as a component, probably expressed by placing an instance of class Y as an attribute in every object of class X.
HAS-A 关系是指一个类 X，它有一个类 Y 作为组件，可能通过将类 Y 的实例作为属性放置在类 X 的每个对象中来表示。
The IS-A relationship refers to a class W which is a class Z, probably because class W is a subclass of class Z, or has class Z somewhere in its inheritance graph. Code which knows how to deal with instances of class Z should be able to handle instances of class W with no code changes required.
IS-A 关系指的是类 W，它是类 Z，可能是因为类 W 是类 Z 的子类，或者在其继承图中的某处有类 Z。知道如何处理 Z 类实例的代码应该能够处理 W 类实例而无需更改代码。
