C ++指针练习

Question

I found the following complete exercise about C++ pointers on the web, but I can't still understand how it works.

#include <iostream>
#include <cstring>

using namespace std;

void reverse(char *s, int n) {
    char *first = &s[0];
    char *last = &s[n-1];
    int k = 0;

    while(first < last){
        char temp = *first;

        *first++ = *last;
        *last-- = temp;

        k++;
    }
}


int main() {
    int n;
    char str[] = "Hello";
    cout << str << endl << endl;
    n = strlen(str);
    reverse(str,n);
    cout << str << endl;
    return 0;
}

The part that I really can't understand is

*first++ = *last;
*last-- = temp;

Answer 1

Pointers are basically addresses in memory, for this specific case they point to addresses in memory where you can find characters. *first is the value (ie: the character) which is at the address and with ++ and -- you increment or decrement the pointer, thus walking through the memory to point to the next/previous character.

*first++ = *last; is evaluated as:

1. Put in the memory address/location where first points the value which is at the memory location where last points
2. increment first , so that it points to the next address.

This is equivalent to:

*first = *last;
first ++;

*last-- = temp; is evaluated as:

1. Put in the memory address/location where last points the value of temp
2. Decrement last , so that it points to the previous address

I leave to you as an exercise to find out with which two operations is this equivalent :)

Answer 2

*first = *last; // copy data from last to first
++first; // point to the next item
*last = temp; // copy data from temp to last
--last; // point to the previous item