在 Golang 中将基类型转换为派生类型

Question

I'm not entirely sure how to ask this question - I have a type that I 'derived' from another using the standard golang type keyword:

type EWKBGeomPoint geom.Point

I want to essentially 'override' the geom.Point Scan() and Value() functions, so I have the following function:

func (g *EWKBGeomPoint) Scan(input interface{}) error {
    gt, err := ewkbhex.Decode(string(input.([]byte)))
    if err != nil {
        return err
    }
    g = gt.(*geom.Point) // error here

    return nil
}

However, when I run this, it has an interface conversion error:

interface conversion: geom.T is *geom.Point, not *models.EWKBGeomPoint

I don't understand why this is the case - I have seen multiple code samples now that allow this behavior. Also it would intuitively seem to me that it should work, because they are basically the same types with all of the same underlying variables and functions.

How can I convert this type to the target type?

Link to go-geom docs

Go version is go1.13.4 linux/amd64

Answer 1

You cannot override functions in Go.

If you define a new type like:

type EWKBGeomPoint geom.Point

Then EWKBGeomPoint is a new type containing the same member fields of geom.Point , but none of its methods.

If you define a new type by embedding:

type EWKBGeomPoint struct {
   geom.Point
}

Then, EWKBGeomPoint is a new type embedding a geom.Point . All member variables and functions of geom.Point are also defined for EWKBGeomPoint .

If you use your declaration, you can convert the result:

g = (*models.EWKBGeomPoint)(gt.(*geom.Point))

If you use type embedding, you can assign it to the embedded point:

g.Point = *gt.(*geom.Point)

However, either case you did not override the Scan function, you defined a Scan function for the new type.

Answer 2

您只能输入 assert接口中的内容，因此您别无选择，只能使用gt.(*geom.Point) ，但您可以转换结果：

g = (*models.EWKBGeomPoint)(gt.(*geom.Point))