Assigment in argument list

Question

why is the following result y=10 not y=30.
is the assigment redirected to non existing y argument and ignored?

using namespace std;

int main() {
int y=10, x=0;
cout<<sizeof(y=y+20)<<endl;
cout<<y;

return 0;
}

Answer 1

sizeof(expression) doesn't evaluate expression. It's so that you can do sizeof(function()) without causing side effects.