golang接口符合性编译类型检查

Question

I see the following statements from camlistore(http://code.google.com/p/camlistore/source/browse/pkg/cacher/cacher.go).

var (
        _ blobref.StreamingFetcher = (*CachingFetcher)(nil)
        _ blobref.SeekFetcher      = (*CachingFetcher)(nil)
        _ blobref.StreamingFetcher = (*DiskCache)(nil)
        _ blobref.SeekFetcher      = (*DiskCache)(nil)
)

I understand that no variables are created and the statements ensure compiler checks that CachingFether implements public functions of StreamingFetcher and SeekFetcher. RHS portion uses a pointer constructor syntax with a nil parameter. What does this syntax mean in Go language ?

Answer 1

(*T)(nil) is a Conversion. In this case it stands for a typed nil, ie. the same value which, for example

var p *T

has before assigning anything to it.

The standard syntax of a conversion is T(expr), but the priority of the * would bind it wrongly in

*T(expr)

This syntax means dereferencing the return value of function T with one argument expr. That's why the conversion has an alternative syntax:

(T)(expr)

where T can of course be *U. Therefore

(*U)(expr)

is the generalized form of what you see in the Camlistore repository.