reverse_iterator reverse iterator

In the previous article   C++ iterator (and reverse iterator)

, we saw the basic usage of iterators.

Now let's look at a strange phenomenon

Unexpected subtraction operation

What does the unsigned number 0 minus 1 become? ? It is known that size_t is generally of unsigned long long type.

#include <iostream>
using namespace std;

int main(void)
{
    size_t i = 0;
    --i;
    cout << i << endl;

    return 0;
}

The above program output:

Why 18446744073709551615? This number is actually 2^64-1.

Binary is: 11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111

That is, the complement of -1. Computer numerical types are represented by two's complement codes .

According to the definition of "Computer Composition Principles" Tang Shuofei 2nd Edition 6.1.2 Unsigned Numbers 6.1.3 Signed Numbers' Complement Code. We can know that the complement of -1 is a binary number with 64 bits all being 1.

In computers, subtraction is calculated using the complement of addition. The result of 0-1 is [0] complement + [-1] complement = 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 + 11111111 1111 1111 11111111 11111111 11111111 11111111 11111111 11111111 = 11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111

size_t type cannot be iterated in reverse order

Consider the following program, what is the output?

#include <iostream>
#include <vector>
using namespace std;

void test_reverse_iterate_by_index(void)
{
    cout << "reverses iterate vector by index:";
    
    vector<int> arr{123};

    for (size_t i = arr.size()-1; i >= 0 ; --i)
    {
        cout << arr[i] << " ";
    }
    arr.rbegin();
    cout << endl;
}

int main(void)
{
    test_reverse_iterate_by_index();

    return 0;
}

Actually the program will crash

Why is this happening?

As mentioned before, unsigned numbers have no negative numbers and all binary bits represent positive integers.

When i subtracts 1 from 0, through the previous analysis, we can know the result of 0-1: 11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111 As an unsigned number, when compared with 0, it is obviously greater than 0.

This causes the program to access arr[11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111], which is obviously out of bounds.

That is, the program pops up Assert failed: out of range.

Reverse iterator reverse_iterator

Therefore, when we need to traverse the container in reverse order, the standard library provides a reverse iterator that allows us to iterate the container conveniently and safely:

#include <iostream>
#include <vector>
using namespace std;
 
int main()
{
	vector<int> a{ 0,1,2,3,4,5,6,7,8,9 };
	//rbegin() 获取逆序迭代器的第一个迭代器；
	//rend() 获得逆序迭代器的最后一个迭代器的下一个位置（哨兵）
	for (auto itr = a.rbegin(); itr != a.rend(); ++itr)
	{
		cout << *itr << " ";
	}
	return 0;
}

Program output:

 Reverse iterator reference:

cplusplus.com/reference/vector/vector/rbegin/