April 2018
Intermediate to advanced
303 pages
4h 54m
Chinese
Content preview from Effective Modern C++（中文版）
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lambda
表达式
｜
225
在前面，我曾提到，对于
std::bind
了无经验的程序员会感觉占位符（例如，
_1
，
_2
等）
看起来像天书一样。不过，可不仅仅只有占位符的行为如此佶屈聱牙。假设我们有一
个函数用来制作
Widget
型别对象的压缩副本，
enum class CompLevel { Low, Normal, High };
//
压缩等级
Widget compress(const Widget& w,
//
制作
w
的
CompLevel lev);
//
压缩副本
然后我们想要创建一个函数对象，这样就可以指定特定的
Widget
型别对象
w
的压缩
级别了。运用
std::bind
，可以创建出这么一个对象：
Widget w;
using namespace std::placeholders;
auto compressRateB = std::bind(compress, w, _1);
这里，当我们把
w
传递给
std::bind
时，然后加以存储，以供未来让
compress
调用时
使用。它存储的位置是在对象
compressRateB
内，但它以哪一种方式存储的：按值，
还是按引用呢？这两者是泾渭分明的，因为如果
w
在对
std::bind
的调用动作与对
compressRateB
调用动作之间被修改了，如果采用按引用方式存储，那么存储起来的
w
的值也会随之修改，而如果采用按值存储，则存储起来的
w
值不会改变。
答案揭晓，
w
是按值存储的。
注
1
可是，了解答案的唯一途径，就是牢记
std::bind ...
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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.