August 2021
Intermediate to advanced
450 pages
5h 35m
Chinese
Content preview from C++语言导学（原书第2版）
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7.4.1 表达式折叠
为了简化简单可变参数模板的实现，C++17提供了一种遍历参数包中元素的有限形式。例如：
在本例中，sum（）可以接受任意数目、任意类型的实参。假设sum（）真的将实参相加，我们就可以编写像下面这样的代码：
sum的函数体使用了表达式折叠：
在这里，（v+...+0）表示以0为初始值，将v中的所有元素相加。第一个累加的元素是“最右”元素（下标最大的元素）：（v[0]+（v[1]+（v[2]+（v[3]+（v[4]+0）））））。即，累加从右边的0开始。这被称为右折叠（right fold）。我们也可以使用左折叠（left fold）：
现在，第一个累加的元素是“最左”元素（下标最小的元素）：（（（（（0+v[0]）+v[1]）+v[2]）+v[3]）+v[4]）。即，从左边的0开始。 ...
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