For an expression x of interface type and a type T, the primary
expression
x.(T)
asserts that x is not nil and that the value stored in x is of
type T. The notation x.(T) is called a type assertion.
More precisely, if T is not an interface type, x.(T) asserts that the
dynamic type of x is identical to the type T. In this case, T must
implement the (interface) type of x; otherwise the type assertion is
invalid since it is not possible for x to store a value of type T. If
T is an interface type, x.(T) asserts that the dynamic type of x
implements the interface T.
If the type assertion holds, the value of the expression is the value
stored in x and its type is T. If the type assertion is false, a
run-time panic occurs. In other words, even though the dynamic type of
x is known only at run time, the type of x.(T) is known to be T in a
correct program.
var x interface{} = 7 // x has dynamic type int and value 7
i := x.(int) // i has type int and value 7
type I interface { m() }
var y I
s := y.(string) // illegal: string does not implement I (missing method m)
r := y.(io.Reader) // r has type io.Reader and y must implement both I and io.Reader
A type assertion used in an assignment or initialization of the special
form
v, ok = x.(T)
v, ok := x.(T)
var v, ok = x.(T)
yields an additional
untyped boolean value. The value of ok is true if the assertion holds.
Otherwise it is false and the value of v is the zero value for type T.
No run-time panic occurs in this case.