使用shared_ptr的开销和实现 [英] Overhead and implementation of using shared_ptr

问题描述

简介：我正在处理多线程代码，我必须在两个线程之间共享动态分配的对象。为了使我的代码更清洁（更少容易出错），我想明确删除每个线程中的对象，这就是为什么我想使用 shared_ptr 。

第一个问题：

我想知道 - > ;在运行时， shared_ptr 中的运算符有一些额外的开销（例如大于 unique_ptr ）。对象我在说的通常是longlife实例只复制一次后创建（当我在线程之间分配），然后我只访问这些对象的方法和字段。

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第二个问题：

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在libstdc ++中优化的 shared_ptr 有多好？它总是使用互斥或​​利用原子操作（我专注于x86和ARM平台）？

解决方案

第一个问题：使用 operator->

$ b b

我看到的所有实现在 shared_ptr< T> 中有一个本地缓存 T * operator-> 具有与使用堆栈local T * ：mutex / atomics

我希望libstdc ++在x86平台上使用原子，无论是通过标准设施还是特定的g ++内在函数（在旧版本中）。我相信Boost的实现已经这样做了。

但是，我不能评论ARM。

>注意：C ++ 11引入移动语义，许多副本在使用 shared_ptr 时自然避免。

注意：请阅读 shared_ptr 此处，您可以使用对 shared_ptr （ const ）的引用），以避免大多数副本/销毁一般，所以这些的性能不太重要。

Short introduction: I am working on multithread code and I have to share dynamically allocated objects between two threads. To make my code cleaner (and less error-prone) I want to explicitly "delete" objects in each thread and that's why I want to use shared_ptr.

First question:

I want to know if implementation of -> operator in shared_ptr has some extra overhead (e.g. larger then unique_ptr) during run time. Objects I am talking about are usually longlife instances copied only once after creation (when i distribute them between threads), then I only access these objects' methods and fields.

I am aware, that shared_ptr only protect reference counting.

Second question:

How well are shared_ptr optimized in libstdc++? Does it always use mutex or take advantage of atomic operations (I focus on x86 and ARM platforms)?

解决方案

First question: using operator->

All the implementations I have seen have a local cache of T* right in the shared_ptr<T> class so that the field is on the stack, operator-> has thus a comparable cost to using a stack local T*: no overhead at all.

Second question: mutex/atomics

I expect libstdc++ to use atomics on x86 platform, whether through standard facilities or specific g++ intrinsics (in the older versions). I believe the Boost implementation already did so.

I cannot, however, comment on ARM.

Note: C++11 introducing move semantics, many copies are naturally avoided in the usage of shared_ptr.

Note: read about correct usage of shared_ptr here, you can use references to shared_ptr (const or not) to avoid most of the copies/destruction in general, so the performance of those is not too important.

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