使用常规迭代器向后迭代,或使用reverse_iterator? [英] Use a regular iterator to iterate backwards, or struggle with reverse_iterator?
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问题描述
我最近学到了使用C ++中的反向迭代器的正确方法(特别是当你需要删除一个时)。 (请参见此问题和这一个。)
I recently learned about the right way to work with reverse iterators in C++ (specifically when you need to erase one). (See this question and this one.)
这是你应该做的:
typedef std::vector<int> IV;
for (IV::reverse_iterator rit = iv.rbegin(), rend = iv.rend();
rit != rend; ++rit)
{
// Use 'rit' if a reverse_iterator is good enough, e.g.,
*rit += 10;
// Use (rit + 1).base() if you need a regular iterator e.g.,
iv.erase((rit + 1).base());
}
但是我认为 不要这样做,不符合标准,正如MooingDuck指出的):
But I think thought this is much better (Don't do this, not standards compliant, as MooingDuck points out):
for (IV::iterator it = iv.end(), begin = iv.begin();
it-- != begin; )
{
// Use 'it' for anything you want
*it += 10;
iv.erase(it);
}
缺点:
-
你告诉我。发生了什么问题? - 这是不符合标准的,因为MooingDuck点出来。
You tell me. What's wrong with it?- It's not standards compliant, as MooingDuck points out. That pretty much overrules any of the possible advantages below.
优点:
- 使用熟悉的惯用法用于反向for-loops
- 不必记住(或解释)
+1
- 减少键入
- 适用于std :: list:
it = il.erase ;
- 如果您删除元素,则不必调整迭代器。
- 不必重新计算begin迭代器
- Uses a familiar idiom for reverse for-loops
- Don't have to remember (or explain) the
+1
- Less typing
- Works for std::list too:
it = il.erase(it);
- If you erase an element, you don't have to adjust the iterator
- If you erase, you don't have to recompute the begin iterator
推荐答案
反向迭代器的原因是标准算法不知道如何对集合进行反向迭代。例如:
The reason for reverse iterators is that the standard algorithms do not know how to iterate over a collection backwards. For example:
#include <string>
#include <algorithm>
std::wstring foo(L"This is a test, with two letter a's involved.");
std::find(foo.begin(), foo.end(), L'a'); // Returns an iterator pointing
// to the first a character.
std::find(foo.rbegin(), foo.rend(), L'a').base()-1; //Returns an iterator
// pointing to the last A.
std::find(foo.end(), foo.begin(), L'a'); //WRONG!! (Buffer overrun)
使用任何迭代器产生更清晰的代码。
Use whichever iterator results in clearer code.
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