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

问题描述

我习惯于进行 Java 编程，在编程时您从不需要真正考虑指针.但是，目前我正在用 C++ 编写程序.在创建具有其他类成员的类时，什么时候应该使用指针，什么时候不应该使用?例如，我什么时候想要这样做:

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;
}

与此相反:

class Foo {
    Bar* b;
}

推荐答案

从避免使用指针开始.

在以下情况下使用它们:

Use them when:

• 您想使用 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 类只需要能够访问它.
• 您想推迟 Bar 对象的构造(即，您想在构造 Foo 之后 创建它).
• 在您的业务逻辑中，Bar 对象可能根本不存在；您也可以在 Java 中使用 null.但是，请查看 boost::optional 也是.
• Bar 实际上是一个基类，你需要实例是多态的.
• 您碰巧使用的工具包更喜欢将 GUI 小部件显示为指针.示例可能包括(但当然不限于)wxWidgets 和 GLUI.

• 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.

在任何这些情况 (*) 中，首先使用智能指针，例如 boost::shared_ptr.否则，您迟早可能会忘记释放内存.一旦您知道自己在做什么，请逐案考虑哪种指针类型最好.

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.

(*) 任何情况——可能除了关于 GUI 小部件的项目符号；在这种情况下，您的工具包很可能也会为您管理资源

(*) 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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