golang的"..."备忘

1. 用于数组: 表示长度与元素个数相同.

在golang中数组的长度是类型的一部分,不同长度,不同类型.

2. 用于参数: 用于形参表示可变参数. 用于实参表示直接传递. 具体解释参数见官方文档:

传递可变参数时:

(1) 如果实参后不跟..., 则在底层创建与形参类型相同的slice,然后将实参赋值后传递.

(2) 如果实参后跟..., 则不在底层创建与形参类型相同的slice,而是直接将实参传递给形参.

/ref/spec#Passing_arguments_to_..._parameters

Passing arguments to ... parameters

If f is variadic with a final parameter p of type ...T, then within f the type of p is equivalent to type []T. If f is invoked with no actual arguments for p, the value passed to p is nil. Otherwise, the value passed is a new slice of type []T with a new underlying array whose successive elements are the actual arguments, which all must be assignable to T. The length and capacity of the slice is therefore the number of arguments bound to p and may differ for each call site.

Given the function and calls

func Greeting(prefix string, who ...string)
Greeting("nobody")
Greeting("hello:", "Joe", "Anna", "Eileen")

within Greeting, who will have the value nil in the first call, and []string{"Joe", "Anna", "Eileen"} in the second.

If the final argument is assignable to a slice type []T, it may be passed unchanged as the value for a ...T parameter if the argument is followed by .... In this case no new slice is created.

Given the slice s and call

s := []string{"James", "Jasmine"}Greeting("goodbye:", s...)

within Greeting, who will have the same value as s with the same underlying array.