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Why does this code have different outputs if pointers are incremented differently c++

December 29, 2021

#include <iostream>
using namespace std;
int main() {
  int num=10;
  int *ptr=NULL;
  ptr=&num;
  num=(*ptr)++; //it should increase to 11
  num=(*ptr)++; //it should increase to 12 but im getting 10
                //if i dont initialize num and just use (*ptr)++ it gives me 11
  cout<<num<<endl;
    return 0;
}

I want to know why is this happening and why am I getting 10 as output.

>Solution :

why is this happening

Because you are using post-increment operator instead of pre-increment operator.

Replace (*ptr)++ with:

num = ++(*ptr);//uses pre-increment operator

And you will get 12 as output at the end of the program which can be seen here.

Alternative solution

You can also just write (*ptr)++; without doing assignment to num. So in this case code would look like:

int main() {
  int num=10;
  int *ptr=NULL;
  ptr=&num;
  (*ptr)++; //no need for assignment to num
  (*ptr)++; //no need for assignment to num
                
  cout<<num<<endl;
    return 0;
}