15.析构函数

　　析构器只适用于类类型，当一个类的实例被释放之前，析构器会被立即调用。析构器用关键字deinit来标示.

1.析构过程

析构器是在实例释放发生前被自动调用。

子类继承了父类的析构器，并且在子类析构器实现的最后，父类的析构器会被自动调用。

即使子类没有提供自己的析构器，父类的析构器也同样会被调用。

因为直到实例的析构器被调用后，实例才会被释放，所以析构器可以访问实例的所有属性。

语法：在类的定义中，每个类最多只能有一个析构器，而且析构器不带任何参数。

deinit
{
//执行析构过程
}

2.示例

//Bank类管理一种虚拟硬币，确保流通的硬币数量永远不可能超过 10,000。在游戏中有且只能有一个Bank存在
class Bank
{
static var coinsInBank = 10000;

static func vendCoins(numberOfCoinsToVend: Int) -> Int
{
let retValue = min(numberOfCoinsToVend, coinsInBank);
coinsInBank -= retValue;
return retValue;
}

static func receiveCoins(coins: Int)
{
coinsInBank += coins;
}
}

class Player
{
var coinsInPurse: Int;

init(coins: Int)
{
coinsInPurse = Bank.vendCoins(coins);
}

func winCoins(coins: Int)
{
coinsInPurse += Bank.vendCoins(coins);
}

deinit
{
Bank.receiveCoins(coinsInPurse);
}
}

var playerOne: Player? = Player(coins: 100);
print("Player have \(playerOne!.coinsInPurse) coins, Bank have \(Bank.coinsInBank)");  //"Player have 100 coins, Bank have 9900\n"
playerOne?.winCoins(2000);
print("Player have \(playerOne!.coinsInPurse) coins, Bank have \(Bank.coinsInBank)");  //"Player have 2100 coins, Bank have 7900\n"
playerOne = nil;
print("Player have \(playerOne?.coinsInPurse) coins, Bank have \(Bank.coinsInBank)");  //"Player have nil coins, Bank have 10000\n"