C ++ - 什么时候应该在类中使用指针成员 [英] C++ - when should I use a pointer member in a class

问题描述

在学习C ++（和Direct3D，但是前一段时间）时，我一直困惑的是，你应该在类中使用指针成员。例如，我可以使用非指针声明：

One of the thing that has been confusing for me while learning C++ (and Direct3D, but that some time ago) is when you should use a pointer member in a class. For example, I can use a non-pointer declaration:

private:
    SomeClass instance_;

或者我可以使用指针声明

Or I could use a pointer declaration

private:
   Someclass * instance_

在构造函数中使用new（）。

And then use new() on it in the constructor.

我知道如果SomeClass可以派生自另一个类，一个COM对象或是一个ABC，那么它应该是一个指针。

I understand that if SomeClass could be derived from another class, a COM object or is an ABC then it should be a pointer. Are there any other guidelines that I should be aware of?

推荐答案

指针具有以下优点：

a）你可以做一个延迟初始化，这意味着在第一次实际使用之前只需要短暂的初始化/创建对象。

a) You can do a lazy initialization, that means to init / create the object only short before the first real usage.

b）设计：如果你为外部类类型的成员使用指针，你可以在你的类上面放置一个向前的声明，因此不需要在你的头中包含该类型的头部 - 而不是包括第三方头部在您的.cpp中，这有利于减少编译时间，并通过包含太多其他标头来防止副作用。

b) The design: if you use pointers for members of an external class type, you can place a forward declaration above your class and thus don't need to include the headers of that types in your header - instead of that you include the third party headers in your .cpp - that has the advantage to reduce the compile time and prevents side effects by including too many other headers.

class ExtCamera;  // forward declaration to external class type in "ExtCamera.h"

class MyCamera {
public: 
  MyCamera() : m_pCamera(0) { }

  void init(const ExtCamera &cam);

private:
   ExtCamera  *m_pCamera;   // do not use it in inline code inside header!
};

c）指针可以随时删除 - 所以你可以更多地控制生命周期，创建一个对象 - 例如在失败的情况下。

c) A pointer can be deleted anytime - so you have more control about the livetime and can re-create an object - for example in case of a failure.

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