Swift 3 access of private properties in a struct from a extension

From the Swift's book:

Private access restricts the use of an entity to the enclosing declaration. Use private access to hide the implementation details of a specific piece of functionality when those details are used only within a single declaration.

So a private variable is only accessible within the set of curly brackets where it's defined. Swift's Access Control has never been like those of C++ , Java or C#.

With release of Swift 4 this has been changed if the extension is implemented within the same file. You can refer the doc here Private Member in Extension

Example below is from Swift 4

protocol SomeProtocol {
    func doSomething()
}
struct SomeStruct {
    private var privateVariable = 12
}
extension SomeStruct: SomeProtocol {
    func doSomething() {
        print(privateVariable)
    }
}