何时使用指针和何时不使用？ [英] When to use pointers and when not to?

问题描述

我习惯于做Java编程，在编程时你永远不需要考虑指针。但是，目前我正在用C ++编写程序。当做类有其他类的成员时，我应该使用指针，我什么时候不应该？例如，我什么时候想这样做：

  class Foo {
 Bar b; 
} 
  

与此相反：

$ b b

  class Foo {
 Bar * b; 
} 
  

解决方案

/ p>

在以下情况下使用它们：

• 您要使用 Pimpl idiom ，或抽象工厂 Bar 实例实际上由程序的其他部分管理，而 Foo code>类只需要能够访问它。


• 您要推迟 Bar 对象即之后创建

。
在您的业务逻辑中， code> Bar 对象可能不存在;你也可以在Java中使用 null 。不过，请查看 boost :: optional 。 / li>

• Bar 实际上是一个基类，您需要该实例为多态。


• 你碰巧使用一个工具包，喜欢将GUI小部件作为指针。示例可以包括（但当然不限于） wxWidgets 和 GLUI 。

在任何情况下智能指针，例如 boost :: shared_ptr 。否则，你很可能会忘记释放内存，迟早。一旦你知道你在做什么，考虑一下一个例子什么指针类型是最好的。

（*）任何case - 除了，可能是关于GUI窗口小部件;在这种情况下，您的工具包最有可能为您管理资源

I'm used to doing Java programming, where you never really have to think about pointers when programming. However, at the moment I'm writing a program in C++. When making classes that have members of other classes, when should I use pointers and when should I not? For example, when would I want to do this:

class Foo {
    Bar b;
}

As opposed to this:

class Foo {
    Bar* b;
}

解决方案

Start by avoiding pointers.

Use them when:

• You want to use the Pimpl idiom, or an abstract factory.
• The Bar instance is actually managed by some other part of your program, whereas the Foo class just needs to be able to access it.
• You want to postpone the construction of the Bar object (i.e., you want to create it after constructing Foo).
• In your business logic, the Bar object may not exist at all; you would use null also in Java. However, check out boost::optional as well.
• Bar is actually a base class, and you need the instance to be polymorphic.
• You happen to be using a toolkit that prefers to present GUI widgets as pointers. Examples could include (but are certainly not limited to) wxWidgets and GLUI.

In any of these cases (*), start by using a smart pointer, such as boost::shared_ptr. Otherwise, you are likely to forget to deallocate the memory, sooner or later. Once you know what you are doing, consider case-by-case what pointer type is best.

(*) any case – except, probably, the bullet regarding GUI widgets; in this case, your toolkit would most probably manage the resources for you as well

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