What does passing by copy mean?

When passing by copy (pass by value) to a function, a copy of the variable's value is passed, not a reference to the original. Changes made to the parameter inside the function do not affect the original variable outside the function.

For structures and enumerations in Swift, passing by copy is used by default. For classes, passing by reference is used, but in this case, a copy of the reference is passed, not the object itself.

Example in Swift:

// Customer structure
struct Customer {
    var name: String
}

// Function that modifies a Customer parameter
func changeCustomerName(customer: Customer) {
    var modifiedCustomer = customer // Creates a copy of the structure
    modifiedCustomer.name = "Jane"
    print("Inside function: \(modifiedCustomer.name)") // Output: Inside function: Jane
}

var myCustomer = Customer(name: "John")
print("Before function: \(myCustomer.name)") // Output: Before function: John

changeCustomerName(customer: myCustomer)

print("After function: \(myCustomer.name)") // Output: After function: John - the original value did not change

Key aspect: the function works with a local copy of the data.