int - Multiplication of two integers in C++

Question

I have a pretty basic question, but I am not sure if I understand the concept or not. Suppose we have:

int a = 1000000;
int b = 1000000;
long long c = a * b;

When I run this, c shows negative value, so I changed also a and b to long long and then everything was fine. So why do I have to change a and b, when their values are in range of int and their product is assigned to c (which is long long)?

I am using C/C++

Answer 1

The ints are not promoted to long long before multiplication, they remain ints and the product as well. Then the product is cast to long long, but too late, overflow has struck.

Having one of a or b long long should work as well, as the other would be promoted.