Type assertions
A type assertion provides access to an interface value's underlying concrete value.
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.
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.