October 2015
Beginner to intermediate
400 pages
14h 44m
English
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7.10 Type Assertions
A type assertion
is an operation applied to an interface value.
Syntactically, it looks like x.(T),
where x is an expression of an interface type
and T is a type, called the “asserted” type.
A type assertion checks that the dynamic type of its operand matches
the asserted type.
There are two possibilities.
First, if the asserted type T is a concrete type, then the type
assertion checks whether x’s dynamic type is identical to
T.
If this check succeeds, the result of the type assertion is x’s
dynamic value, whose type is of course T.
In other words, a type assertion to a concrete type extracts the
concrete value from its operand.
If the check fails, then the operation panics.
For example:
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