在Go中使用未导出的子结构创建结构的实例

Question

I am trying to manually create an instance of the type ReactionAddedEvent given here, in nlopes' Go Slack library. However, the sub-type reactionItem is unexported, which leads me to receive the error ./bot_test.go:111: cannot refer to unexported name slack.reactionItem when trying to instantiate the object.

Here is my code:

m := &slack.ReactionAddedEvent{ Item: &slack.reactionItem{ File: &slack.File{ Preview: "Test", URLPrivate: "http://google.com", }, }, Reaction: "white_check_mark", }

When I remove the identifier &slack.reactionItem from line 2 in that snippet, I get instead the error: ./bot_test.go:112: missing type in composite literal, obviously.

Is there any way for me to instantiate an object of this type with the parameters I need?

Answer 1

First, if slack here refers to the nlopes library, the slack.ReactionAddedEvent structure's Item field is not a pointer, so you can't store an address of a slack.reactionItem struct into that field anyway. Second, the File field of slack.reactionItem is a string, not a structure.

Third, even if the above isn't/weren't the case, if the types are not exported, but the fields themselves are, you can't assemble the structure in a single literal. Instead, you'll have to manually set those fields after creating the structure variable:

m := &slack.ReactionAddedEvent{Reaction: "white_check_mark"}
m.Item.File.Preview = "Test"
m.Item.File.URLPrivate = "http://google.com"

But again, if you're using the nlopes library, that won't work because the File field isn't actually a structure: https://github.com/nlopes/slack/blob/master/websocket_reactions.go

And fourth, if the type isn't exported, that's probably a good sign that you shouldn't be manipulating objects of that type. In this case, in the nlopes library, those structures are only intended to be used for unmarshalling and then handling events from JSON messages.