What happens if I use vector::begin() instead of std::back_inserter(vector) for output of set_intersection?(如果我使用 vector::begin() 而不是 std::back_inserter(vector) 作为 set_intersection 的输出会发生什么?)
问题描述
我一直在使用高度简洁和直观的 C++ 语法来查找两个已排序 vector 的交集并将结果放入第三个
vector
:
矢量a,b,c;//...std::set_intersection(a.begin(),a.end(),b.begin(),b.end(),std::back_inserter(c));
这应该将 c
设置为交集(a
,b
),假设 a
和 b
已排序.
但是如果我只使用 c.begin()
会怎样(我以为我在某处看到了一个例子,这就是我这样做的原因):
std::set_intersection(a.begin(),a.end(),b.begin(),b.end(),c.开始());
set_intersection
在该参数处需要一个 OutputIterator
.我认为的标准只要求 c.begin()
返回一个 forward iterator
,我认为它可能是也可能不是 OutputIterator
.
反正c.begin()
的代码是在clang下编译的.
在标准下保证会发生什么?如果编译成功,可能会发生什么——也就是说,当 c.begin()
返回的迭代器最终递增到向量的末尾,并尝试访问指向的元素时到,必须/可能发生什么?在这种情况下,一个符合要求的实现是否可以默默地扩展向量,以便 begin()
实际上是一个附加的 OutputIterator
,就像 back_inserter
一样?p>
我问这个主要是为了了解标准如何与迭代器一起工作:真正发生了什么,所以我可以在使用 STL 时超越复制和粘贴.
输出迭代器的重要要求是它在范围内有效且可写[out, out+
输出大小)
.
传递 c.begin()
将导致值被覆盖,这仅在容器 c
包含足够的元素以覆盖时才有效.想象一下 c.begin()
返回一个指向大小为 0 的数组的指针 - 然后你会在编写 *out++ = 7;
时看到问题.
back_inserter
将每个赋值添加到 vector
(通过 push_back
)并提供一种简洁的制作方法STL 算法扩展了一个范围——它适当地重载了用于迭代器的运算符.
这样
std::set_intersection(a.begin(),a.end(),b.begin(),b.end(),c.开始());
一旦 set_intersection
将某些内容写入其输出迭代器,即当 a
和 b
的集合交集不是时,
调用未定义的行为空.
<块引用>在这种情况下,符合标准的实现是否可以默默地扩展向量,以便 begin() 实际上是一个附加的 OutputIterator
,就像 back_inserter
一样?
当然.这是未定义的行为.(这是一种幽默的方法,告诉你,你甚至不应该考虑使用它,无论对任何实现的影响.)
I have been using the highly concise and intuitive C++ syntax for finding the intersection of two sorted vector
s and putting the result in a third vector
:
vector<bar> a,b,c;
//...
std::set_intersection(a.begin(),a.end(),b.begin(),b.end(),
std::back_inserter(c));
This should set c
to intersection(a
,b
), assuming a
and b
are sorted.
But what if I just use c.begin()
(I thought I saw an example somewhere of this, which is why I did):
std::set_intersection(a.begin(),a.end(),b.begin(),b.end(),
c.begin());
set_intersection
expects an OutputIterator
at that parameter. The standard I believe requires only that c.begin()
return a forward iterator
, which I suppose might or might not be an OutputIterator
.
Anyway, the code with c.begin()
compiled under clang.
What is guaranteed to happen under the standard? If this compiles, what is likely to happen - that is, when the iterator returned by c.begin()
is eventually incremented past the end of the vector, and an attempt is made to access the element pointed to, what must/may happen? Can a conforming implementation silently extend the vector in this case, so that begin()
is in fact an appending OutputIterator
like back_inserter
is?
I'm asking this mainly to understand how the standard works with iterators: what's really going on, so I can move beyond copy-and-paste in using the STL.
The important requirement for an output iterator is that it be valid and write-able for the range [out, out+
size of output)
.
Passing c.begin()
will lead to the values being overwritten which only works if the container c
holds enough elements to overwrite. Imagine that c.begin()
returns a pointer to an array of size 0 - then you'll see the problem when writing *out++ = 7;
.
back_inserter
adds every assigned value to a vector
(via push_back
) and provides a concise way of making the STL-algorithms extend a range - it overloads the operators that are used for iterators appropriately.
Thus
std::set_intersection(a.begin(),a.end(),b.begin(),b.end(),
c.begin());
invokes undefined behavior once set_intersection
writes something to its output iterator, that is, when the set intersection of a
and b
isn't empty.
Can a conforming implementation silently extend the vector in this case, so that begin() is in fact an appending
OutputIterator
likeback_inserter
is?
Of course. It's undefined behavior. (This is a humorous approach of telling you that you shouldn't even consider using this, no matter the effects on any implementation.)
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