c - How does comma operator work during assignment?

Question

int a = 1;
int b = (1,2,3);
cout << a+b << endl; // this prints 4

1. Is (1,2,3) some sort of structure in c++ (some primitive type of list, maybe?)
2. Why is b assigned the value 3? Does the compiler simply take the last value from the list?

Answer 1

Yes, that's exactly it: the compiler takes the last value. That's the comma operator, and it evaluates its operands left-to-right and returns the rightmost one. It also resolves left-to-right. Why anyone would write code like that, I have no idea :)

So int b = (1, 2, 3) is equivalent to int b = 3. It's not a primitive list of any kind, and the comma operator , is mostly used to evaluate multiple commands in the context of one expression, like a += 5, b += 4, c += 3, d += 2, e += 1, f for example.