why std::deque::rbegin() - rend() gives a negative number?

[Peng Yu]

Hi,

I don't understand why rbegin() -rend() gives a negative number.

Since rbegin() + 1 gives the one before the last element, I think
rbegin() - rend() should give a positive number.

Thanks,
Peng

#include <deque>
#include <iostream>

int main() {
std::deque<doubleq;
q.push_back(1);
q.push_front(0);
q.push_back(2);
q.push_back(3);
std::cout << q[0] << std::endl;
std::cout << q.rbegin() - q.rend() << std::endl;//gives a negative
number?

std::cout << *(q.rbegin() + 1) << std::endl;// gives the one before
the last element
}

[zhangyw80]

On 9ÔÂ12ÈÕ, ÏÂÎç12ʱ06·Ö, Peng Yu <[e-mail]:

Hi,

I don't understand why rbegin() -rend() gives a negative number.

Since rbegin() + 1 gives the one before the last element, I think
rbegin() - rend() should give a positive number.

Thanks,
Peng

#include <deque>
#include <iostream>

int main() {
std::deque<doubleq;
q.push_back(1);
q.push_front(0);
q.push_back(2);
q.push_back(3);
std::cout << q[0] << std::endl;
std::cout << q.rbegin() - q.rend() << std::endl;//gives a negative
number?

std::cout << *(q.rbegin() + 1) << std::endl;// gives the one before
the last element

}- Òþ²Ø±»ÒýÓÃÎÄ×Ö -

- ÏÔʾÒýÓõÄÎÄ×Ö -

that's right, rbegin() - rend() gives a negative number indeed
given a vector that have 5 elements,
rbegin() refers to the last element, and rend() refers to the
first element, so
rbegin() + 4 = rend()
then rbegin - rend() = -4
the result(-4) is negative

[Juha Nieminen]

Peng Yu wrote:

I don't understand why rbegin() -rend() gives a negative number.

Because they return a regular range. rbegin() returns the beginning of
the range and rend() returns the end of the range. rend()-rbegin() (for
random access iterators) returns the size of the range. That's how
iterator ranges are defined.

Just because reverse iterators happen to traverse the container
backwards doesn't change anything. It's still just a regular range, like
any other.

If they behaved like you expect, that could potentially break some
algorithms which expect end-begin to return the proper size of the range.

[zhangyw80]

my fault, as for my example,
rbegin() - rend() should be -5
I thought rend() is the first element.
actually, rend() refer to "null" element before the first element