C++ 为什么我不能取消引用迭代器？
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why can't I dereference an iterator?
提问by SirYakalot
If I have a vector of objects (plain objects, not pointers or references), why can't I do this?
如果我有一个对象向量（普通对象，而不是指针或引用），为什么我不能这样做？
Object* object;
object = vectorOfObjects.end();
or
或者
object = &vectorOfObjects.end();
or
或者
object = &(*vectorOfObjects.end());
also the same question if 'object' was a reference.
如果 'object' 是一个参考，同样的问题。
回答by jalf
They are three separate errors:
它们是三个独立的错误：
object = vectorOfObjects.end();
won't work because end()returns a an iterator, and objectis a pointer. Those are generally distinct types (A vector canuse raw pointers as iterators, but not all implementations do it. Other containers require special iterator types).
不会工作，因为end()返回一个迭代器，并且object是一个指针。这些通常是不同的类型（向量可以使用原始指针作为迭代器，但并非所有实现都这样做。其他容器需要特殊的迭代器类型）。
object = &vectorOfObjects.end();
doesn't work because you take the address of the returned iterator. That is, you get a pointer to an iterator, rather than a pointer to an Object.
不起作用，因为您采用了返回的迭代器的地址。也就是说，您获得的是指向迭代器的指针，而不是指向Object.
object = &(*vectorOfObjects.end());
doesn't work because the enditerator doesn't point to a valid element. It points one past the end of the sequence. And so, it can't be dereferenced. You can dereference the last element in the sequence (which would be --vectorOfObjects.end()), but not an iterator pointing pastthe end.
不起作用，因为end迭代器未指向有效元素。它指向序列末尾的一个。因此，它不能被取消引用。您可以取消引用序列中的最后一个元素（这是--vectorOfObjects.end()），但不是一个迭代器，指向过去的结束。
Finally, the underlying problem/confusion might be that you think an iterator can be converted to a pointer. In general, it can't. If your container is a vector, you're guaranteed that elements are allocated contiguously, like in an array, and so a pointer will work. But for, say, a list, a pointer to an element is useless. It doesn't give you any way to reach the nextelement.
最后，潜在的问题/困惑可能是您认为迭代器可以转换为指针。一般来说，不能。如果您的容器是向量，则可以保证元素是连续分配的，就像在数组中一样，因此指针将起作用。但是对于 a 来说 list，指向元素的指针是没有用的。它没有给你任何到达下一个元素的方法。
回答by Luchian Grigore
Because .end()returns an iterator, not an Object, which isn't even a valid member of the vector.
因为.end()返回一个迭代器，而不是一个Object，它甚至不是向量的有效成员。
Even using begin(), which returns a valid object, you'd need the following:
即使使用begin()返回有效对象的 ，您也需要以下内容：
The correct way would be:
正确的方法是：
std::vector<Object> vec;
std::vector<Object>::iterator it;
//....
it = vec.begin();
Object o = *it;
回答by dasblinkenlight
This is because by convention enddoes not point to a valid location in the container: it sits at a location one past the end of the container, so dereferencing it is illegal.
这是因为按照惯例end并不指向容器中的有效位置：它位于容器末尾的位置，因此取消引用它是非法的。
The situation with begin()is different: you can dereference it when v.begin() != v.end()(i.e. when the container is not empty):
情况begin()不同：您可以在以下情况下取消引用它v.begin() != v.end()（即当容器不为空时）：
object = *vectorOfObjects.begin();
If you need to access the last element, you can do this:
如果您需要访问最后一个元素，您可以这样做：
vector<object>::const_iterator i = vectorOfObjects.end();
i--;
cout << *i << endl; // Prints the last element of the container
Again, this works only when your vector contains at least one element.
同样，这仅在您的向量包含至少一个元素时才有效。
回答by msgmash.com
Object* means a pointer to an Object, where a pointer is a memory address. But the .end() method returns an Iterator, which is an object itself, not a memory address.
Object* 表示指向 Object 的指针，其中指针是内存地址。但是 .end() 方法返回一个 Iterator，它是一个对象本身，而不是一个内存地址。
