Reference counting

Reference counting的优势:

1、相对简单、容易实现

无处不在的counting

计数，并且当count归0的时候，就执行某个操作，这在computer science中有着非常广泛的application:

Automatic resource management

本节标题的含义是: 自动资源管理。

需要注意的是，本节标题使用的是"resource"而不是"memory"，它的含义是: Reference counting不仅仅可以用于管理memory，还可以用于管理更加广泛的resource。下面是一些例子:

1、shared_ptr

当reference个数为0的时候，则析构object；

2、reader write lock

当reader个数为0的时候，则唤醒writer；

当writer个数为0的时候，则唤醒reader；

3、Memory reclamation

在lockfree programming中，它有着非常广泛的应用；

wikipedia Reference counting

In computer science, reference counting is a programming technique of storing the number of references, pointers, or handles to a resource, such as an object, a block of memory, disk space, and others.

Advantages and disadvantages

NOTE: 下面的这一段，着重描述的是:

1、Reference counting VS tracing garbage collection

Objects are reclaimed as soon as they can no longer be referenced

The main advantage of the reference counting over tracing garbage collection is that objects are reclaimed as soon as they can no longer be referenced, and in an incremental fashion, without long pauses for collection cycles and with clearly defined lifetime of every object.

Simplest forms of memory management

Reference counting is also among the simplest forms of memory management to implement.

Multiuse

NOTE: 多用途

It also allows for effective management of non-memory resources such as operating system objects, which are often much scarcer than memory (tracing garbage collection systems use finalizers for this[citation needed], but the delayed reclamation may cause problems). Weighted reference counts are a good solution for garbage collecting a distributed system.

wikipedia Dealing with reference cycles

NOTE: 第一次阅读的时候，并没有搞清楚reference cycle的含义，原文中的这样的一段话：

the Cocoa framework, for instance, recommends using "strong" references for parent-to-child relationships and "weak" references for child-to-parent relationships.

当parent-to-child和child-to-parent都使用strong reference的时候，则两者相互引用，则两者都不可能的reference count都不可能成为0，则两者最终都无法被析构。

stackexchange Why are reference-counting smart pointers so popular?

NOTE: 主要对比reference counting 和 garbage collection

A

NOTE: 这个回答着重强调的是reference counting的优势

Some advantages of reference counting over garbage collection:

1、Low overhead.

2、More predictable behavior.

3、Simpler.

4、Standard.

Implementation

spdlog

spdlog使用std::shared_ptr来实现reference counting。

那如何使用std::atomic来实现呢？

参见 Implement-reference-count-use-std-atomic 章节。

TODO

mortoray What is reference counting?