附加到接口切片

Question

I'm writing a document generator. There is a DocumentItem interface - this is part of the document.

type DocumentItem interface {
    compose() string
}

For example, a document consists of paragraphs and tables.

type Paragraph struct {
    text string
}
type Table struct{}

The Paragraph and Table types correspond to the DocumentItem interface.

func (p *Paragraph) compose() string {
    return ""
}

func (t *Table) compose() string {
    return ""
}

The Document type contains the content []*DocumentItem field.

type Document struct {
    content []*DocumentItem
}

I'm looking for a way that would allow NewParagraph() and NewTable() functions to create the necessary data types and add them to the content field.

func (d *Document) NewParagraph() *Paragraph {
    p := Paragraph{}
    d.content = append(d.content, &p)
    return &p
}

func (d *Document) NewTable() *Table {
    t := Table{}
    d.content = append(d.content, &t)
    return &t
}

I use an slice of interface pointers in order to be able to modify the data in the corresponding variables after they are included in the document.

func (p *Paragraph) SetText(text string) {
    p.text = text
}

func main() {
    d := Document{}
    p := d.NewParagraph()
    p.SetText("lalala")
    t := d.NewTable()
    // ...    
}

But I get compiler errors:

cannot use &p (type *Paragraph) as type *DocumentItem in append
cannot use &t (type *Table) as type *DocumentItem in append

If I cast the types to an DocumentItem interface, I will lose access to specific functions, which in the case of one type may differ from the other. For example, add text to a paragraph and add a row of cells and then add text to cell for the table.

Is it possible at all?

Full example at https://play.golang.org/p/uJfKs5tJ98

Answer 1

Don't use pointer to interface, only a slice of interfaces:

content []DocumentItem

If the dynamic value wrapped in the interface value is a pointer, you lose nothing, you will be able to modify the pointed object.

This was the only thing that had to be changed. To verify, I added printing at the end:

fmt.Printf("%+v", p)

Output (try it on the Go Playground):

&{text:lalala}