April 2018
Intermediate to advanced
303 pages
4h 54m
Chinese
Content preview from Effective Modern C++（中文版）
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60
｜
第
3
章
要把那一行伪代码变换成实际代码，有两种方式（有关
std::forward
的信息，参见
条款
25
）：
T localObject
(
std::forward<Ts>(params)...
)
;
//
采用小括号
T localObject
{
std::forward<Ts>(params)...
}
;
//
采用大括号
所以，考虑一下对应的调用代码：
std::vector<int> v;
…
doSomeWork<std::vector<int>>(10, 20);
如果
doSomeWork
在创建
localObject
使用了小括号，结果会得到一个包含
10
个元素
的
std::vector
。如果
doSomeWork
使用了大括号，结果会得到一个包含
2
个元素的
std::vector
。哪个是对的呢？
doSomeWork
的
作者不可能下这个判断，只有调用者才
有决定权。
这正是标准库函数
std::make_unique
和
std::make_shared
（参见条款
21
）所面临的
问题。而这些函数解决问题的办法是在内部使用了小括号，并把这个决定以文档的形
式广而告之，作为其接口的组成部分。
注
1
要点速记
•
大括号初始化可以应用的语境最为宽泛，可以阻止隐式窄化型别转换，还
对最令人苦恼之解析语法免疫。
•
在构造函数重载决议期间，只要有任何可能，大括号初始化物就会与带有
std::initializer_list
型别的形参相匹配，即使其他重载版本有着貌似更
加匹配的形参表。
•
使用小括号还是大括号，会造成结果大相径庭的一个例子是：使用两个实 ...
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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.