嵌入结构以覆盖方法

Question

TL;DR See playground link at bottom.

I have methods defined on a Manager struct which contains a Context. The Managers are versioned, allowing a new version to only define functions which have changed, and automatically use the functions from the old version if they are not redefined.

type Context struct { ... }

type Manager1 struct{
    Context Context
}

type Manager2 struct {
    Manager1
    Context Context
}

When calling a function on Manager2 which is not defined on Manager2 the Context is nil. Is there a way to do this where the context will be available?

This example demonstrates the problem better than I can explain it: http://play.golang.org/p/gFe6GgUKEJ

Answer 1

You're misusing embedding. The thing is, you've defined Context on Manager1, in spite of that you've redefined it on each subsequent type. In Manager3 you're setting the value for it's Context instance. When Hello() is called, it's defined on Manager2 and accesses it's Context instance which doesn't have a value. Check out this example to demonstrate that http://play.golang.org/p/XebShA9ap4

Money line is: m3 = Manager3{Manager2: Manager2{Context: Context{Value: "testing3"}}}

As you can see, if I instantiate the Manager2 instance embedded in Manager3 and set it's Context value it gets printed. I would recommend changing your types so that Context is only defined on Manager1 and then use syntax like that in my example when you initialize your types.

EDIT: To put the design discussed in comments in writing, you would change your types to this;

type Context struct {
    Value string
}

type Manager1 struct {
    Context Context
}

type Manager2 struct {
    Manager1
}

type Manager3 struct {
    Manager2
}

Delete the implementation of Hello() on Manager2 altogether. Then update your composite-literal initialization to this;

m1 := Manager1{Context: Context{Value: "testing1"}}
m2 := Manager2{Manager1: Manager1{Context: Context{Value: "testing2"}}}
m3 := Manager3{Manager2: Manager2{Manager1: Manager1{Context: Context{Value: "testing3"}}}}