Pointer arithmetic and arrays in C++

Question

I tried to make a new char array and add to it chars. I wanted it to do by pointer. I know that is possible to move pointers in array by pointer arithmetic , so I wanted to use it. However it doesn't work and I don't know really why. Any idea why i can't use pointer arithemtic in array?

#include <math.h>
#include <iostream>
using namespace std;
void print (char *k){
 
  while(*k!=0)cout<<*k++;

}

int main(){

   int max = 128;
   char* temp1 = new char[max];
   *temp1 = 0;
   char ch;
   while (cin.get(ch)) {
        if (ch=='\n') break;
        *++temp1 = 0;
        *--temp1 = ch;
        temp1++;
      
  }
  
  print(temp1);  

}

The problem is that pointer arithmetic did work. And as a result, you can't print the string because the pointer no longer points to the beginning of the array. You need one pointer that always points to the beginning of the array, and another pointer to add characters to the array. — user3386109
– user3386109, Commented Mar 18, 2021 at 7:46

Jabberwocky · Accepted Answer · 2021-03-18 07:49:49Z

2

This is very convoluted

*++temp1 = 0;
*--temp1 = ch;
temp1++;

The idiomatic way would be this:

*temp1++ = ch;
*temp1 = 0;

But most importantly print(temp1); is wrong, because temp1 does not point to the string but it points to the end of the string.

You want this:

int main() {
  int max = 128;
  char* thestring = new char[max];
  char* temp1 = thestring;
  *temp1 = 0;
  char ch;
  while (cin.get(ch)) {
    if (ch == '\n') break;
    *temp1++ = ch;
    *temp1 =0;
  }

  print(thestring);
}

or even simpler:

int main() {
  int max = 128;
  char* thestring = new char[max];
  char* temp1 = thestring;
  char ch;
  while (cin.get(ch)) {
    if (ch == '\n') break;
    *temp1++ = ch;
  }

  *temp1 = 0;

  print(thestring);
}

edited Mar 18, 2021 at 7:49

answered Mar 18, 2021 at 7:44

Jabberwocky

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1 Comment

Jarod42 Over a year ago

Even simpler, remove the new: char s[128]{}; char* p = s; /*..*/ print(s);.

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