Any one could explain me what is the meaning of past-the-end. Why we call end() function past-the-end?
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Milad Khajavi
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5 Answers
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The functions begin() and end() define a half open range([begin, end)), which means:
The range includes first element but excludes the last element. Hence, the name past the end.
The advantage of an half open range is:
It avoids special handling for empty ranges. For empty ranges,
begin()is equal toend().It makes the end criterion simple for loops that iterate over the elements: The loops simply continue as long as
end()is not reached
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The illustration smells like Josuttis :) – fredoverflow Mar 06 '13 at 16:30
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1Not just `begin()` and `end()`; generally, `[begin, end)` defines a **sequence** of values, regardless of where the iterators come from or where the values are held. – Pete Becker Mar 06 '13 at 16:58
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63. The number of elements in the range is `end - begin` (for random-access iterators), not some expression with an annoying +1 in it. – Steve Jessop Mar 06 '13 at 17:06
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Because it doesn't point to the last element of a container, but to somewhere past the last element of a container.
If you dereference end() it results in undefined behaviour.
alestanis
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Like interval in mathematics, stl uses [begin, end).
That's why we could write for (auto it = v.begin(); it != v.end(); ++it)
lostyzd
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1Yes! The first answer that doesn't talk about containers, which are not required to create sequences. – Pete Becker Mar 06 '13 at 17:00
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But so is `[begin,end]`; and then there also are `(a,b]`, `(a,b)`. Please elaborate, what's so special that you see in math about the `[a,b)` compared to other intervals? – Hi-Angel Mar 25 '19 at 23:32
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@Hi-Angel [Why numbering should start at zero](https://www.cs.utexas.edu/users/EWD/transcriptions/EWD08xx/EWD831.html) – lostyzd Nov 21 '19 at 11:24
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Literally, because it points one past the end of the array.
It is used because that element is empty, and can be iterated to, but not dereferenced.
int arry[] = {1, 2, 3, 4, /* end */ };
^^^^^^^
std::end(arry) would point here.
Tony The Lion
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Not just arrays; the `end` iterator points past the end of the target sequence, regardless of where the the iterators come from or where the values are held. – Pete Becker Mar 06 '13 at 16:59
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Adding another point to the above correct answers. This was also done to be compatible with arrays. For example in the code below:
char arr[5];
strcpy(arr, "eakgl");
sort(&arr[0], &arr[5]);
This will work fine.
Instead if you had given :
sort(&arr[0], &arr[4]);
it would miss sorting the last character.
This also helps to represent empty containers naturally.
programmerjake
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VNarasimhaM
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