（* DefaultHandler）（nil）是什么意思，其中DefaultHandler是一个结构？ [重复]

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• Specs: What's the purpose of the blank identifier in variable assignment? [duplicate] 1 answer

I can understand whole last line of code meaning: It provides a static (compile time) check that *DefaultHandler satisfies the EasygateHandler interface. But I cannot understand what's the meaning of (*DefaultHandler)(nil) and what's the difference with (*DefaultHandler)()

type DefaultHandler struct {
    log logrus.FieldLogger
    svc *service.DefaultResolver
}

var _ easygate.EasygateHandler = (*DefaultHandler)(nil)

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Answer 1

The expression (*DefaultHandler)(nil) is a conversion from an untyped nil to a zero value of type *DefaultHandler.

Conversions are of the form T(x) where T is a type and x is a value that can be converted to type T. In this example, T is *DefaultHandler and x is nil. The parenthesis around *DefaultHandler are required to distinguish the conversion to a pointer type from a dereference of a a conversion to the non-pointer type.

The expression (*DefaultHandler)() is not valid Go syntax.

The value &DefaultHandler{} can also be used if DefaultHandler is a type with composite literal syntax. The conversion pattern works for all types.