Go语言中结构体嵌入与接口实现的疑问：为何自定义错误类型无法复用嵌入error的接口实现？

Great question! This is a common point of confusion with Go's embedding system—let's break down why your two examples behave so differently, and clear up how embedding works with structs vs. interfaces.

First, Let's Recap Your Working Admin/Notifier Example #

In your first code snippet, Admin embeds a User struct. The notify() method is defined on *User (a pointer receiver). When you pass &admin to SendNotification, Go automatically promotes the notify() method from *User to *Admin's method set. This works because:

• Embedding a struct type (like User) into another struct (Admin) allows the outer struct (and its pointer) to inherit methods from the embedded struct (and its pointer) when the receiver type matches.
• Since &admin gives you a *Admin, Go looks at the embedded User field, takes its pointer (&admin.User), and uses its notify() method to satisfy the Notifier interface.

Now, Why Your Error Example Fails #

Your norgateMathError struct embeds an err error field—this is an interface type, not a struct. Go's embedding rules work very differently here:

• Embedding an interface into a struct does not automatically promote the methods of the interface's underlying value to the outer struct. Instead, embedding an interface is a way to declare that your struct should implement that interface—but you still have to provide the method implementations yourself.
• The error interface requires an Error() string method. Your embedded err field has this method, but it belongs to the err instance itself, not to the norgateMathError type. Go doesn't treat the field's methods as part of the outer struct's method set.

The Fix: Implement Error() Yourself #

To make *norgateMathError satisfy the error interface, you need to explicitly define the Error() method on it. You can absolutely reuse the embedded err's implementation—you just have to wrap it:

func (n *norgateMathError) Error() string {
    // Reuse the embedded error's message while adding your context
    return fmt.Sprintf("a norgate math error occurred: %v %v %v", n.lat, n.long, n.err)
}

Or if you just want to pass through the embedded error's message directly:

func (n *norgateMathError) Error() string {
    return n.err.Error()
}

Core Difference Between Embedding Structs vs. Interfaces #

To sum up the key distinction:

• Embedding a struct: Go promotes methods from the embedded struct (and its pointer) to the outer struct (and its pointer) when the receiver type aligns. This is how your Admin example works.
• Embedding an interface: This is a way to compose interface requirements into your struct, but it does not inherit method implementations from the interface's value. You must define the required methods on your struct itself—even if you just delegate to the embedded field.

内容的提问来源于stack exchange，提问作者Володимир Христенко