传递对象并将其捕获到接口中时无法访问函数{}

Question

Here's what I'm trying to do:

package products

ProductManager struct {
    products []*Product
    index    int64
}

func NewPM() *ProductManager {
    return &ProductManager{}
}

func (p *ProductManager) List() []*Product {
    return p.products
}

---

var productManager = products.NewPM()

func main() {
    api := Api{}
    api.Attach(productManager, "/products")
}

func (api Api) Attach(rm interface{}, route string) {
    // Apply typical REST actions to Mux.
    // ie: Product - to /products
    mux.Get(route, http.HandlerFunc(index(rm)))
}

func index(rm interface{}) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // allRecords := rm.List() - DOESN'T WORK. No method found.
        result := reflect.TypeOf(rm) // Works, shows me the correct type.
        method := result.Method(0).Name // Works as well! Shows me a function on the type.
        w.Write([]byte(fmt.Sprintf("%v", method)))
    }
}

Any idea on why I can't call the List() function on my productManager object? I can see the Type's functions and even get the right Type name in the index() handler.

Answer 1

Dead simple: rm is an empty interface variable, so it does not have any methods, that's exactly what interface{} means: No methods. If you know that rm's dynamic type is *ProductManager you can type assert it (but than you could pass a *ProductManager) Maybe you'll have to type switch on rm in the longer run. (BTW: using interface{} is almost always a bad idea.)