Golang断言Type assertions

断言type assertion提供了access to an interface value's underlying concrete value.

直接断言

t := i.(T)

This statement asserts that the interface value i holds the concrete type T and assigns the underlying T value to the variable t.

If i does not hold a T, the statement will trigger a panic.直接断言会导致panic。

测试断言

To test whether an interface value holds a specific type, a type assertion can return two values: the underlying value and a boolean value that reports whether the assertion succeeded.

t, ok := i.(T)

If i holds a T, then t will be the underlying value and ok will be true.

If not, ok will be false and t will be the zero value of type T, and no panic occurs.

Note the similarity between this syntax and that of reading from a map.

示例：

package main

import "fmt"

func main() {
	var i interface{} = "hello"

	s := i.(string)
	fmt.Println(s)

	s, ok := i.(string)
	fmt.Println(s, ok)

	f, ok := i.(float64)
	fmt.Println(f, ok)

	f = i.(float64) // panic
	fmt.Println(f)
}

输出

hello
hello true
0 false
panic: interface conversion: interface {} is string, not float64

goroutine 1 [running]:
main.main()
	/tmp/sandbox227310573/prog.go:17 +0x1fe

配合Switch

另外也可以配合switch语句进行判断：

package main

import "fmt"

func findType(i interface{}) {
    switch x := i.(type) {
    case int:
        fmt.Println(x, "is int")
    case string:
        fmt.Println(x, "is string")
    case nil:
        fmt.Println(x, "is nil")
    default:
        fmt.Println(x, "not type matched")
    }
}

func main() {
    findType(10)      // int
    findType("hello") // string

    var k interface{} // nil
    findType(k)

    findType(10.23) //float64
}

运行结果：

10 is int
hello is string
<nil> is nil
10.23 not type matched

References:
https://studygolang.com/articles/3314