Swift Learning--swift explains the specific basis (vii)

Implicitly unwrapped optionals implicit parsing options

Mentioned above. Optional means that the constant or variable "has no value". The optional if declaration is used to infer whether a value exists, assuming that there is a value for the analysis value.

Sometimes in the program architecture, the first time is assigned a value. The ability to determine an optional always has a value.

In such a case. It is inefficient to infer and parse an optional value every time, because it is possible to determine that it always has values.

The optional of such a type is defined as an implicit parsing optional (implicitly unwrapped optionals).

Declare an implicit parsing option by changing the question mark (String?) that you want to use as an optional type to an exclamation point (string!).

Implicit parsing is useful when it is possible to determine after the first assignment that the value is always there. Implicit parsing is primarily used in the construction of classes in Swift, please refer to cyclic strong references between class instances.

An implicit parsing option is, in fact, a common option, but can be used as a non-optional and does not require parsing to get an optional value every time. The following example shows the difference between an optional string and an implicitly resolved optional string:

Let possiblestring:string?
= "An optional string." println (possiblestring!)//requires an exclamation mark to access its value//prints ' an optional string. ' Let Assumedstr ing:string! = "An implicitly unwrapped optional string."  println (assumedstring)  //No exclamation mark are needed to access their value//prints "an implicitly unwrapped optional String.

You can choose implicit parsing as an option that can be parsed on its own initiative.

All you have to do is declare the exclamation point at the end of the type, not at the end of the optional name each time the value is taken.

Note: Suppose you try to take a value when implicit parsing is optional without a value. Will trigger an execution-time error. And you add an exclamation point to the normal optional value.

You can still infer that implicit parsing is optional as a normal alternative to whether it includes values:

If assumedstring {    println (assumedstring)}//prints "an implicitly unwrapped optional string.

You can also use implicit parsing in an optional binding to examine and parse its value:

If let definitestring = assumedstring {    println (definitestring)}//prints "an implicitly unwrapped optional string.

Note: Do not use the implicit parsing option if a variable may become nil after that. Suppose you need to infer whether the life cycle of a variable is nil if. Please use a normal optional type.

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Swift Learning--swift explains the specific basis (vii)