为什么 std::for_each 是非修改序列操作?
我刚刚在 C++ 标准中读到 std::for_each
是一个非修改序列操作,以及 find
、search
和很快.这是否意味着应用于每个元素的函数不应修改它们?这是为什么?可能会出现什么问题?
I just read in the C++ standard that std::for_each
is a non-modifying sequence operation, along with find
, search
and so on. Does that mean that the function applied to each element should not modify them? Why is that? What could possibly go wrong?
这是一个示例代码,其中修改了序列.你能看出它有什么问题吗?
Here is a sample code, where the sequence is modified. Can you see anything wrong with it?
void foo(int & i)
{
i = 12;
}
int main()
{
std::vector<int> v;
v.push_back(0);
std::for_each(v.begin(), v.end(), foo);
// v now contains 12
}
我怀疑这只是一个解释问题,但我想听听您对此的看法.
I suspect this to be just an interpretation issue, but I wanted to have your opinion about that.
PS:我知道我可以使用 std::transform
而不是 for_each
,但这不是重点.
PS: I know I could use std::transform
instead of for_each
, but that's not the point.
推荐答案
参见 这个缺陷报告他们说
LWG 认为标准中没有任何内容禁止修改序列元素的函数对象.问题是 for_each 位于名为非变异算法"的部分中,标题可能会令人困惑.非规范性说明应澄清这一点.
The LWG believes that nothing in the standard prohibits function objects that modify the sequence elements. The problem is that for_each is in a secion entitled "nonmutating algorithms", and the title may be confusing. A nonnormative note should clarify that.
还要注意这个.
他们似乎称之为非修改",因为 for_each 本身并没有明确修改序列的元素.
They seem to call it "non-modifying" because for_each itself does not exlicitly modify the elements of the sequence.
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