How to create slice of struct using reflection?

Question

I need to create a slice of struct from its interface with reflection.

I used Reflection because do not see any other solution without using it.

Briefly, the function receives variadic values of Interface.

Then, with reflection creates slice and passes it into another function.

Reflection asks to type assertion

SliceVal.Interface().(SomeStructType)

But, I cannot use it.

Code in playground http://play.golang.org/p/EcQUfIlkTe

The code:

package main

import (
    "fmt"
    "reflect"
)

type Model interface {
    Hi()
}

type Order struct {
    H string
}

func (o Order) Hi() {
    fmt.Println("hello")
}

func Full(m []Order) []Order{
    o := append(m, Order{H:"Bonjour"}
    return o
}

func MakeSlices(models ...Model) {
    for _, m := range models {
        v := reflect.ValueOf(m)
        fmt.Println(v.Type())
        sliceType := reflect.SliceOf(v.Type())
        emptySlice := reflect.MakeSlice(sliceType, 1, 1)
        Full(emptySlice.Interface())
    }
}
func main() {
    MakeSlices(Order{})
}

Not_a_Golfer · Accepted Answer · 2015-03-10 10:52:23Z

4

You're almost there. The problem is that you don't need to type-assert to the struct type, but to the slice type.

So instead of

SliceVal.Interface().(SomeStructType)

You should do:

SliceVal.Interface().([]SomeStructType)

And in your concrete example - just changing the following line makes your code work:

Full(emptySlice.Interface().([]Order))

Now, if you have many possible models you can do the following:

switch s := emptySlice.Interface().(type) {
case []Order:
    Full(s)
case []SomeOtherModel:
    FullForOtherModel(s)
// etc
}

edited Mar 10, 2015 at 10:52

answered Mar 10, 2015 at 10:18

Not_a_Golfer

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5 Comments

maksadbek Over a year ago

thanks, I appreciate this ansert. as the funtion receives variadic variables, how can I set type assertion when I do not know which struct slice is being created?

Not_a_Golfer Over a year ago

@user2660766 How can you call Full for a specific type if you have many possible types? You can simply do a type switch and call a separate function for each possible model.

maksadbek Over a year ago

It was just example function for test

Not_a_Golfer Over a year ago

@user2660766 see my expanded answer, I've added an exmaple on how to route to multiple functions based on the model

Nick Charney Kaye Over a year ago

Any ideas on how this might work without actually knowing the struct type to assert? stackoverflow.com/questions/40474682/…

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