调用struct方法

Question

This code works fine:

feedService := postgres.FeedService{}
feeds, err := feedService.GetAllRssFeeds()

But this code gives me error:

feeds, err = postgres.FeedService{}.GetAllRssFeeds()

controllers\feed_controller.go:35: cannot call pointer method on postgres.FeedService literal controllers\feed_controller.go:35: cannot take the address of postgres.FeedService literal

Why this two pieces of code is not equal ?

Here is a struct declaration:

type FeedService struct {

}

func (s *FeedService) GetAllRssFeeds() ([]*quzx.RssFeed, error) {

Answer 1

Your FeedService.GetAllRssFeeds() method has pointer receiver, so a pointer to FeedService is needed to call this method.

In your first example you use a short variable declaration to store a FeedService struct value in a local variable. Local variables are addressable, so when you write feedService.GetAllRssFeeds() after that, the compiler will automatically take the address of feedService and use that as the receiver value. It is a shorthand for:

feeds, err := (&feedService).GetAllRssFeeds()

It is in Spec: Calls:

If x is addressable and &x's method set contains m, x.m() is shorthand for (&x).m().

In your second example you don't create a local variable, you just use a struct composite literal, but by itself it is not (automatically) addressable, so the compiler cannot obtain a pointer to FeedService value to be used as the receiver, and hence cannot call the method.

Note that it is allowed to take the address of a composite literal explicitly, so the following also works:

feeds, err := (&postgres.FeedService{}).GetAllRssFeeds()

This is in Spec: Composite literals:

Taking the address of a composite literal generates a pointer to a unique variable initialized with the literal's value.

See related questions:

What is the method set of `sync.WaitGroup`?

Calling a method with a pointer receiver by an object instead of a pointer to it?