#include <iostream> using namespace std; int main() { int num=10; int *ptr=NULL; ptr=# 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.
(*ptr)++ increases num to 11 but returns its previous value (10), because the ++ is postfix.
So with num = (*ptr)++, you are temporarily increasing num to 11, but then (re)assigning it with 10.
It is caused by assigning to num. ++ operator returns old value and then increments. However, then the old value is assigned to num.
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.
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=# (*ptr)++; //no need for assignment to num (*ptr)++; //no need for assignment to num cout<<num<<endl; return 0; } 
(*ptr)++; for each statement.
(*ptr)++; without assigning to num. I have added this at the end of my answer. Check it out.
num=(*ptr)++;exhibits undefined behaviour. Compilers will likely not spot it but, replacing withnum = num++will generate warning : multiple unsequenced modifications to 'num' [-Wunsequenced] with clang-cl (using the C++14 Standard).