使用？分配给swift 3.0中的可选变量？运算符返回nil

var a:Int?

a? = 10

print(a)

这里变量a没有被分配值10.如果是因为’？’运算符,为什么编译器不显示编译错误？

var a:Int?

a = 10

print(a)

好…

? (Optional) indicates your variable may contain a nil value while ! (unwrapper) indicates your variable must have a memory (or value) when it is used (tried to get a value from it) at runtime.

主要区别在于,当optional是nil时,可选链接会正常失败,而当optional可选为nil时,强制解包会触发运行时错误.

var defaultNil : Int?  // declared variable with default nil value
println(defaultNil) >> nil  

var canBeNil : Int? = 4
println(canBeNil) >> optional(4)

canBeNil = nil
println(canBeNil) >> nil

println(canBeNil!) >> // Here nil optional variable is being unwrapped using ! mark (symbol), that will show runtime error. Because a nil optional is being tried to get value using unwrapper

var canNotBeNil : Int! = 4
print(canNotBeNil) >> 4

var cantBeNil : Int = 4
cantBeNil = nil // can't do this as it's not optional and show a compile time error

Here is basic tutorial in detail, by Apple Developer Committee.