shared_ptr的缺点 [英] Disadvantages of shared_ptr

问题描述

通过包含在c ++ 11中的shared_ptr，可以实现半垃圾回收环境。

With shared_ptr included in c++11, one could achieve a semi garbage-collected enviroment. Does the (inflationary?) usage come along with some disadvantages?

我可以想象一个类模型，你可以在其中创建一个类，在该类中，你的类在最后是typedef a shared_ptr以缩写语法。

I could imagine a class model, where you create a class in which you typedef your class at the end as a shared_ptr to abbreviate the syntax.

/////////////////
//// MyClass ////
/////////////////

#include <memory>

class MyClass {
public:
    Myclass();
};

typedef std::shared_ptr<MyClass> SharedMyClass;

///////////////////////
//// Example Class ////
///////////////////////

class Example {
public:
    Example(): myClassObject(new MyClass()) {}
private:
    SharedMyClass myClassObject;
};

推荐答案

当然还有一些缺点：保持引用计数，并且每次复制或销毁一个 shared_ptr 实例时，此引用计数必须递增和递减。如果你的程序使用多个线程，那么引用计数的操作必须以线程安全的方式进行，这会增加一些额外的开销，其大小取决于实现。

Of course there are disadvantages: you require extra memory to maintain a reference count, and every time you copy or destroy a shared_ptr instance this reference count has to be incremented and decremented. If your program uses multiple threads then the manipulation of the reference count has to be done in a thread-safe manner, which can add some additional overhead, the magnitude of which depends on the implementation.

这种开销是否以整体负面影响您的程序取决于您的程序的详细信息。一般来说，你应该只使用 std :: shared_ptr 用于真正需要共享的资源，因为它可能有点任意的对象最后需要它们。在其他情况下，单个所有权点通常更容易维护，因为您确切知道每个资源将活动的时间。

Whether or not this overhead impacts your program in an overall negative way depends on the details of your program. Generally you should only use std::shared_ptr for resources that truly need to be shared, because it can be somewhat arbitrary which object will need them last. In other cases, a single point of ownership is usually easier to maintain because you know exactly how long each resource will be alive.

此外，您需要注意 std :: shared_ptr 不是对象生命周期管理的灵丹妙药。特别是，如果你有两个对象，每个对象都拥有一个 std :: shared_ptr ，那么对象将拥有循环所有权，并且永远不会被自动删除，除非你先中断循环通过在其中一个上调用 reset（）。

Additionally, you need to be aware that std::shared_ptr is not a panacea for object lifetime management. In particular, if you have two objects that each own an std::shared_ptr to the other, the objects have cyclic ownership and will never be automatically deleted unless you first break the cycle by invoking reset() on one of them.

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