This is a type statement . A type statement can be used to:
- get a value of a specific type from an interface type value
- or to get a value of an interface type different from the original one (an interface with a different set of methods is practically not a subset of the original one, since this could simply be obtained with a simple type conversion ).
I quote from the specification:
For an expression x type interface and type T main expression
x.(T)
states that x not nil and that the value stored in x is of type T The notation x.(T) is called a type statement.
More precisely, if T not an interface type, x.(T) claims that the dynamic type x is identical to the type T In this case, T must implement the type (interface) x ; otherwise, a type statement is not valid because x can store a value of type T If T is an interface type, x.(T) claims that the dynamic type x implements the T interface
More precisely, your example is a special form that also tells you whether a type statement is valid. If not, ok will be false , and if the statement is complete, ok will be true .
This special form never panics, unlike the form:
paxPayment = dataObject.(*entities.PassengerPayment)
Which, if dataObject does not contain a value of type *entities.PassengerPayment dataObject will panic.