Swift类型推断需要问号或感叹号

Question

After reading Apple's ARC guide, I'm slowly attempting to get a grasp on retain cycles however what isn't clear to me is swift's type inference requires the optional question mark or forced unwrapping exclamation point when inferring a variable in the global scope of a class.

For example:

import XCTest
@testable import PassionProject

class ItemManagerTests: XCTestCase {

    var sut: ItemManager!

    override func setUp() {
        super.setUp()
        // Put setup code here. This method is called before the invocation of each test method in the class.

        sut = ItemManager()
    }

    override func tearDown() {
        // Put teardown code here. This method is called after the invocation of each test method in the class.
        super.tearDown()
    }

    func tests_ToDoCount_Is_InitiallyZero() {

        XCTAssertEqual(sut.toDoCount, 0)
    }

    func tests_DoneCount_Is_InitiallyZero(){

        XCTAssertEqual(sut.doneCount, 0)
    }
}

If I leave out the question mark or explanation point on the following line, it throws an error about the class not having initializers:

var sut: ItemManager

My question is, isn't type inference just simply saying this variable will be of this type? If so, why is Xcode considering it a property if we haven't given it an initial value? Also second, why does force unwrapping a inferred type compile if we never set its value?

If needed, here is the code for the object we're using as an example and thank you in advance for getting a better grasp:

import Foundation

class ItemManager {

    let toDoCount = 0

    let doneCount = 0

}

Answer 1

That's not type inference. If you declare a variable's type, inference doesn't happen. Inference is all about figuring ( inferring ) out what a type is if you don't say explicitly .

You're having a problem with Swift initializer rules. If you declare that a class has this property:

var sut: ItemManager

Then that's non-optional, which means it must have a value by the time initialization is complete . You're not doing that, so Swift is complaining about your initializers. You can either add an init method that assigns a value or you could declare it and assign a value at the same time-- which might look like this:

var sut: ItemManager = ItemManager()

If you declare it like this:

var sut: ItemManager?

Then it's optional, which means if you don't assign a value then it gets a value of nil. You don't have to assign a value during initialization because it already has one.

Answer 2

Swift, for safety reasons, requires all variables to always hold a value. This prevents that scenario where the value of a variable can be unknown. However, there are still cases in programming where one wants to represent the absence of a value. A great example of this is when performing a search. One would want to be able to return something from the search that indicates that no value was found.

Therefore in Swift,the class members must have a value at the time of declaration.By default a member of a specific type ,say, Int cannot be nil and it does not get a default value.If we know that the value of a variable may be nil then we define it as an Optional.

Now you have three options to provide an initial value to a class member :

1. By initialising it at the time of declaration

2. By providing it a value in the init method

3. By defining it as an optional or unwrapped optional which depends upon the use of the variable