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

From:

zhangyw80@yahoo.com.cn

Newsgroups:

comp.lang.c++

Date:

Thu, 11 Sep 2008 22:00:32 -0700 (PDT)

Message-ID:

<723ec1c1-3d02-44ac-80d2-0d06cf229dbc@b38g2000prf.googlegroups.com>

On 9=D4 12=C8=D5, =CF =CE=E712=CA=B106=B7=D6, Peng Yu <PengYu...@gmail.=
com> wrote:

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<double> q;
  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

}- =D2=FE=B2=D8=B1=BB=D2=FD=D3=C3=CE=C4=D7=D6 -

- =CF=D4=CA=BE=D2=FD=D3=C3=B5=C4=CE=C4=D7=D6 -

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