why the value still can be changed

Question

class C{
var a = 5
}
let c = C()
c.a = 6
c.a = 7
print(c.a)
// why the value still can be changed even though we use let

Answer 1

You don't change the value of your var c, just the value of one of its attributes. That's a big difference. c is still the same object and remains, even if you modify the value of any attribute. OOP basics.

Answer 2

PS: easy cross-check for you, try this:

let c = C() // valid
c.a = 0 // valid
c = C() // failure