区别？而且！在Swift语言？

发布时间：2020-12-14 05:49:09 所属栏目：百科来源：网络整理

导读：参见英文答案 What is the difference between String? and String! (two ways of creating an optional variable)?7个我在线阅读了很多文章并预订了(Apple),但我无法找出它们之间的区别？而且！ Swift语言中的运算符. override func tableView(tableView:

参见英文答案 > What is the difference between String? and String! (two ways of creating an optional variable)?7个
我在线阅读了很多文章并预订了(Apple),但我无法找出它们之间的区别？而且！ Swift语言中的运算符.

override func tableView(tableView: UITableView!,cellForRowAtIndexPath indexPath: NSIndexPath!) -> UITableViewCell!
    {

    }

    override func numberOfSectionsInTableView(tableView: UITableView?) -> Int {
        return 1;
    }

在上面的代码中,第一个func声明tableView为！并在第二个函数tableView中声明为？.

根据Apple官方文档：

When working with optional values,you can write ? before operations
like methods,properties,and subscripting. If the value before the ?
is nil,everything after the ? is ignored and the value of the whole
expression is nil. Otherwise,the optional value is unwrapped,and
everything after the ? acts on the unwrapped value. In both cases,the
value of the whole expression is an optional value.

Once you’re sure that the optional does contain a value,you can
access its underlying value by adding an exclamation mark (!) to the
end of the optional’s name. The exclamation mark effectively says,“I
know that this optional definitely has a value; please use it.” This
is known as forced unwrapping of the optional’s value (…)

Apple Inc.的“Swift编程语言”. iBooks的. https://itun.es/nl/jEUH0.l

（编辑：李大同）

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