April 2018
Intermediate to advanced
303 pages
4h 54m
Chinese
Content preview from Effective Modern C++（中文版）
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202
｜
第
5
章
位域
最后一种完美转发失败情形，是位域被用作函数实参。为考察在实践中如何表现，观
察如下这个可以表示
IPv4
头部的模型：
注
3
struct IPv4Header {
std::uint32_t version:4,
IHL:4,
DSCP:6,
ECN:2,
totalLength:16;
…
};
如果我们被虐了千百遍的函数
f
（转发函数
fwd
万年不变的目标）的声明式中接受
std::size_t
型别的形参，然后用
IPv4Header
对象的，比如说，
totalLength
字段
来
调用
f
吧，编译器会乖乖放行：
void f(std::size_t sz);
//
待调用的函数
IPv4Header h;
…
f
(h.totalLength);
//
没问题
但是，如果是经由
fwd
把
h.totalLength
转发给
f
，就是另一回事了：
fwd
(h.totalLength);
//
错误！
问题在于
fwd
的形参是个引用，而
h.totalLength
是个非
const
的位域。乍听之下，
这也没什么。但是
C++
标准却对于这么个组合以异乎寻常的口吻严加禁止：“非
const
引用不得绑定到位域”，该条禁令倒是有极其充分的理由。位域是由机器字的若干任
意部分组成的（例如，
32
位
int
的第
3
到第
5
个比特），但是这样的实体是不可能有
办法对其直接取址的。我前面曾经提及，在硬件层次，引用和指针本是同一事物。这
么一来，既然没有办法创建指涉到任意比特的指针（ ...
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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.