April 2018
Intermediate to advanced
303 pages
4h 54m
Chinese
Content preview from Effective Modern C++（中文版）
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智能指针
｜
133
从效率的角度来看，
s
td::weak_ptr
和
std::shared_ptr
从本质上来说是一致的。
std::weak_ptr
的对象和
std::shared_ptr
的对象尺寸相同，它们和
std::shared_ptr
使用同样的控制块（参见条款
19
），其构造，析构，赋值操作都包含了对引用计数
的原子操作。这种说法可能会令你惊讶，因为在本条款开头我曾提过，
std::weak_
ptr
不参与引用计数操作。其实我的说法与此有别，我说的是
std::weak_ptr
不干涉对
象的共享所有权，因此不会影响所指涉到的对象的引用计数。实际上控制块里还有第
二个引用计数，
std::weak_ptr
操作的就是这第二个引用计数。更多细节请参考条款
21
。
要点速记
•
使用
std::weak_ptr
来代替可能空悬的
std::shared_ptr
。
•
std::weak_ptr
可能的用武之地包括缓存，观察者列表，以及避免
std::shared_ptr
指针环路。
条款
21
：优先选用
std::make_unique
和
std::make_shared
，而非直接使用
new
让我们以把
std::make_unique
和
std::make_shared
放到同一起跑线作为本条款的开
篇。
std::make_shared
是
C++11
的一部分，但很可惜
std::make_unique
不是。它是
在
C++14
中才加入标准库的。如果你在使用
C++11
，不必担心，写出一个基础版本
的
std::make_unique
易如反掌。请看这段代码： ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.