What does "pass by copy" mean in the context of programming?

"Pass-by-value" means that when a function or method is called, a copy of the variable's value is passed as an argument, not the variable itself. Any changes made to this argument inside the function/method do not affect the original variable.

In Swift, structures (structs), enumerations (enums), and tuples are value types and are passed by copy by default. Classes, closures, and functions are reference types and are passed by reference.

Example of pass-by-copy in Swift:

struct MyStruct {
    var value: Int
}

func modifyStruct(s: MyStruct) {
    var mutableS = s // A copy of s is created
    mutableS.value = 20
    print("Inside modifyStruct: \(mutableS.value)") // 20
}

var originalStruct = MyStruct(value: 10)
print("Before modifyStruct: \(originalStruct.value)") // 10
modifyStruct(s: originalStruct)
print("After modifyStruct: \(originalStruct.value)") // 10 (the original structure remains unchanged)

In this example, the function modifyStruct receives a copy of originalStruct. Changing mutableS.value inside the function does not affect originalStruct.value.

Example of pass-by-reference for comparison (using a class):

class MyClass {
    var value: Int = 0
}

func modifyClass(c: MyClass) {
    c.value = 20
    print("Inside modifyClass: \(c.value)") // 20
}

var originalClass = MyClass()
originalClass.value = 10
print("Before modifyClass: \(originalClass.value)") // 10
modifyClass(c: originalClass)
print("After modifyClass: \(originalClass.value)") // 20 (the original class has changed)

Here, the function modifyClass receives a reference to the originalClass object. Changing c.value through this reference modifies the original object.