April 2018
Intermediate to advanced
303 pages
4h 54m
Chinese
Content preview from Effective Modern C++（中文版）
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168
｜
第
5
章
return w;
}
编译器必须要么省略
w
的复制操作，要么让函数进行特别处理，以与下面这样的代码
等价：
Widget makeWidget()
{
Widget w;
…
return
std::move(w)
;
// w
会作为右值处理，
}
//
原因是复制省略没有实施
上述情况与按值传递的函数形参类似。它们作为函数返回值时，不适合实施复制省略，
但编译器必须在其返回时作为右值处理。以结果论，如果你的代码看起来是这样的：
Widget
makeWidget(
Widget w
)
//
按值传递的形参，
{
//
与函数返回值型别相同
…
return w
;
}
在编译器必须处理上面这段代码，以使它们与以下代码等价：
Widget makeWidget(Widget w)
{
…
return
std::move(w);
//
将
w
作为右值处理
}
这意味着，针对函数中按值返回的局部对象实施
std::move
的操作，不能给编译器
帮上忙（如果不执行复制省略，就必须将局部对象作为右值处理，效果一样），却
可以帮倒忙（可能会排除掉
RVO
的实施机会）。的确存在适合于针对局部变量实施
std::move
的情况（亦即，将其传递给某个函数，并且你确定自己不再会使用该变量），
但是其作为
return
语句的一部分时，要么适合
RVO
，要么返回一个按值形参，从而
并不属于以上的那些情况。
要点速记
•
针对右值引用的最后一次使用实施
std::move
，针对万能引用的最后一次使
用实施
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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.