反复修改本身已被修改的容器的C ++惯用方式 [英] C++ idiomatic way of iterating over a container that itself is being modified

问题描述

我发现自己遍历在循环内被修改的容器X.但是在每次迭代中，都需要未修改的X:

I find myself iterating over a container X that is being modified inside the loop. But in each iteration, the unmodified X is needed:

for (int x : X) { // wrong, container X in iteration should be the "original"
    modify_X();  // X modified here
}

一种解决方案是遍历一个副本

A solution is to iterate over a copy

containerT X_copy(X);
for (int x : X_copy) {
    modify_X();   
}

或

for (int x : containerT {X}) {
    modify_X();   
}

是否有惯用的方法?

推荐答案

问题中的最后一个例子看起来最简单.但是，从C ++ 20开始，您也可以这样做:

The last example in your question looks the simplest. However, from C++20, you could also do this:

for (auto copy = X; int i : copy)
{
  modify_X();
}

这是演示.

请注意，在您的第一个代码段中，这不仅在逻辑上是错误的，而且在您修改要迭代的范围时，还会调用未定义的行为.

Note that in your first snippet, it's not only logically wrong, but it also invokes undefined behavior, as you are modifying a range that you're iterating over.

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