C ++ 11基于范围的for循环：如何忽略值？ [英] C++11 range-based for loop: how to ignore value?
问题描述
我有一个C ++容器，我想运行一个循环与容器中的元素相同的次数。但是我不关心循环中容器中的值。例如：
I have a C++ container and I want to run a loop the same number of times as there are elements in that container. But I do not care about the values in the container during the loop. For example:
for (const auto& dummy : input) {
cout << '.';
}
唯一的问题是 dummy 是一个未使用的变量，我已经指示编译器禁止这些。
The only problem is, dummy is an unused variable and I have instructed the compiler to prohibit those.
我想出的两个不起眼的解决方法是说 void）dummy; c>在循环体中使沉默编译器，或者使用旧式for循环从0到 distance（begin（input），end（input）
Two inelegant solutions I have come up with are to say (void)dummy; in the loop body to silence the compiler, or to use an old-style for loop from 0 to distance(begin(input), end(input)).
我尝试省略变量名，但没有编译（没有大惊喜）。
I tried omitting the variable name but that failed to compile (no big surprise).
我正在使用GCC 4.7.2。
I'm using GCC 4.7.2.
推荐答案
如果你实际想要执行的操作是你在你的例子中写的一个简单的一个，一个更好的方式来实现你想要的是：
If the operation that you actually want to perform is the trivial one that you wrote in your example, a much better way to achieve what you want is:
std::cout << std::string{boost::distance(input), '.'};
否则，如果你只想循环一个范围并对每个元素执行操作，忽略元素值， for_each < algorithm> 正是你想要的。
Otherwise, if you just want to loop over a range and perform an operation for each element, ignoring the element value, the for_each <algorithm> is exactly what you want.
-
使用boost：
Using boost:
#include <boost/range/algorithm.hpp>
boost::for_each(input, [](auto&& /*ignored*/) { /* do stuff */; });
使用STL：
Using the STL:
#include <algorithm>
std::for_each(std::begin(input), std::end(input), [](auto&& /*ignored*/) {
/* do stuff */;
});
请注意， c $ c> break 或 continue ，但你可以 return 行为。
Note that within the lambda you cannot break or continue, but you can return early to achieve the same behavior.
仍然，你应该看看另一个 STL算法，并选择与您的意图最匹配的那个（通常也是最快的）。
Still, you should take a look at the other STL algorithms and choose the one that matches your intent best (it will generally also be the fastest).
这篇关于C ++ 11基于范围的for循环：如何忽略值？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
