Memory Safety

Understanding Conflicting Access to Memory

                  // A write access to the memory where one is stored.

var one = 1



// A read access from the memory where one is stored.

print("We're number \(one)!")
                

Characteristics of Memory Access

                  func oneMore(than number: Int) -> Int {

    return number + 1

}



var myNumber = 1

myNumber = oneMore(than: myNumber)

print(myNumber)

// Prints "2"
                

Conflicting Access to In-Out Parameters

                  var stepSize = 1



func increment(_ number: inout Int) {

    number += stepSize

}



increment(&stepSize)

// Error: conflicting accesses to stepSize
                

                  // Make an explicit copy.

var copyOfStepSize = stepSize

increment(&copyOfStepSize)



// Update the original.

stepSize = copyOfStepSize

// stepSize is now 2
                

                  func balance(_ x: inout Int, _ y: inout Int) {

    let sum = x + y

    x = sum / 2

    y = sum - x

}

var playerOneScore = 42

var playerTwoScore = 30

balance(&playerOneScore, &playerTwoScore)  // OK

balance(&playerOneScore, &playerOneScore)

// Error: conflicting accesses to playerOneScore
                

Conflicting Access to self in Methods

                  struct Player {

    var name: String

    var health: Int

    var energy: Int



    static let maxHealth = 10

    mutating func restoreHealth() {

        health = Player.maxHealth

    }

}
                

                  extension Player {

    mutating func shareHealth(with teammate: inout Player) {

        balance(&teammate.health, &health)

    }

}



var oscar = Player(name: "Oscar", health: 10, energy: 10)

var maria = Player(name: "Maria", health: 5, energy: 10)

oscar.shareHealth(with: &maria)  // OK
                

                  oscar.shareHealth(with: &oscar)

// Error: conflicting accesses to oscar
                

Conflicting Access to Properties

                  var playerInformation = (health: 10, energy: 20)

balance(&playerInformation.health, &playerInformation.energy)

// Error: conflicting access to properties of playerInformation
                

                  var holly = Player(name: "Holly", health: 10, energy: 10)

balance(&holly.health, &holly.energy)  // Error
                

                  func someFunction() {

    var oscar = Player(name: "Oscar", health: 10, energy: 10)

    balance(&oscar.health, &oscar.energy)  // OK

}