Databases
StackOverflow2013
Postsdbo
POHow to pass a dynamically created struct as non-pointer object in Go
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POHow to pass a dynamically created struct as non-pointer object in Go
    primarykey
    data
    text
    <p>I'm trying to dynamically pass parameters to a URL route handler function. I thought it would be possible to use the reflection package to convert a map of values from the URL to the function that has one parameter that happens to be an anonymous struct. I've gotten as far as creating the struct to pass to the handler function but it ends up being a pointer to the struct. If I change the handler function's signature to expect a pointer the created struct ends up being a pointer to a pointer, I think. At any rate, here's the code (the panic follows):</p> <p>Link: <a href="http://play.golang.org/p/vt_wNY1f08" rel="nofollow">http://play.golang.org/p/vt_wNY1f08</a></p> <pre><code>package main import ( "errors" "fmt" "net/http" "reflect" "strconv" "github.com/gorilla/mux" ) func mapToStruct(obj interface{}, mapping map[string]string) error { dataStruct := reflect.Indirect(reflect.ValueOf(obj)) if dataStruct.Kind() != reflect.Struct { return errors.New("expected a pointer to a struct") } for key, data := range mapping { structField := dataStruct.FieldByName(key) if !structField.CanSet() { fmt.Println("Can't set") continue } var v interface{} switch structField.Type().Kind() { case reflect.Slice: v = data case reflect.String: v = string(data) case reflect.Bool: v = string(data) == "1" case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32: x, err := strconv.Atoi(string(data)) if err != nil { return errors.New("arg " + key + " as int: " + err.Error()) } v = x case reflect.Int64: x, err := strconv.ParseInt(string(data), 10, 64) if err != nil { return errors.New("arg " + key + " as int: " + err.Error()) } v = x case reflect.Float32, reflect.Float64: x, err := strconv.ParseFloat(string(data), 64) if err != nil { return errors.New("arg " + key + " as float64: " + err.Error()) } v = x case reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64: x, err := strconv.ParseUint(string(data), 10, 64) if err != nil { return errors.New("arg " + key + " as int: " + err.Error()) } v = x default: return errors.New("unsupported type in Scan: " + reflect.TypeOf(v).String()) } structField.Set(reflect.ValueOf(v)) } return nil } type RouteHandler struct { Handler interface{} } func (h RouteHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) { t := reflect.TypeOf(h.Handler) handlerArgs := reflect.New(t.In(0)).Interface() if err := mapToStruct(handlerArgs, mux.Vars(req)); err != nil { panic(fmt.Sprintf("Error converting params")) } f := reflect.ValueOf(h.Handler) args := []reflect.Value{reflect.ValueOf(handlerArgs)} f.Call(args) fmt.Fprint(w, "Hello World") } type App struct { Router mux.Router } func (app *App) Run(bind string, port int) { bind_to := fmt.Sprintf("%s:%d", bind, port) http.Handle("/", &amp;app.Router) http.ListenAndServe(bind_to, &amp;app.Router) } func (app *App) Route(pat string, h interface{}) { app.Router.Handle(pat, RouteHandler{Handler:h}) } func home(args struct{Category string}) { fmt.Println("home", args.Category) } func main() { app := &amp;App{} app.Route("/products/{Category}", home) app.Run("0.0.0.0", 8080) } </code></pre> <p>Panic:</p> <pre><code>2013/03/28 18:48:43 http: panic serving 127.0.0.1:51204: reflect: Call using *struct { Category string } as type struct { Category string } /usr/local/Cellar/go/1.0.3/src/pkg/net/http/server.go:589 (0x3fb66) _func_004: buf.Write(debug.Stack()) /usr/local/Cellar/go/1.0.3/src/pkg/runtime/proc.c:1443 (0x11cdb) panic: reflect·call(d-&gt;fn, d-&gt;args, d-&gt;siz); /usr/local/Cellar/go/1.0.3/src/pkg/reflect/value.go:428 (0x484ba) Value.call: panic("reflect: " + method + " using " + xt.String() + " as type " + targ.String()) /usr/local/Cellar/go/1.0.3/src/pkg/reflect/value.go:334 (0x47c3a) Value.Call: return v.call("Call", in) /Users/matt/Workspaces/Go/src/pants/pants.go:86 (0x2f36) RouteHandler.ServeHTTP: f.Call(args) /Users/matt/Workspaces/Go/src/pants/pants.go:1 (0x347c) (*RouteHandler).ServeHTTP: package main /Users/matt/Workspaces/Go/src/github.com/gorilla/mux/mux.go:86 (0x5a699) com/gorilla/mux.(*Router).ServeHTTP: handler.ServeHTTP(w, req) /usr/local/Cellar/go/1.0.3/src/pkg/net/http/server.go:669 (0x337b6) (*conn).serve: handler.ServeHTTP(w, w.req) /usr/local/Cellar/go/1.0.3/src/pkg/runtime/proc.c:271 (0xfde1) goexit: runtime·goexit(void) </code></pre>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USpeterSO
    1. USMatt W
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POHow to pass a dynamically created struct as non-pointer object in Go
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThis is probably an abuse of reflection in Go. By doing this, you lose one of Go's most important features - it's type safety. It's hard to get what you're trying to do, but this looks like you really only need a map of http.Handlers.
      singulars
      1. POHow to pass a dynamically created struct as non-pointer object in Go
      1. USYves Junqueira
    2. COCan't quite see what you are trying to do ... but whatever it is it looks like a bad idea. Why does your `Handler` need to be an `interface{}` rather than a func type? Why can't your handler just accept the `map`? ... I would suguest trying to pretend the reflect package doesn't exist for this problem.
      singulars
      1. POHow to pass a dynamically created struct as non-pointer object in Go
      1. USChris Farmiloe
    3. COAbuse? I'd say thats subjective. Bad idea? Subjective as well. The tools are there to achieve what I'm trying to do so whatever. My goal, regardless of your opinion, is to develop an easy to use API for handling URL's with parameters that are part of the URL. Similar to how the Python framework Flask does it.
      singulars
      1. POHow to pass a dynamically created struct as non-pointer object in Go
      1. USMatt W
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload