dirtyCast converts from []Foo to []Bar without copy.

dirtyCast.go

package main

import (
	"encoding/json"
	"fmt"
	"unsafe"
)

// dirtyCast converts from []Foo to []Bar without copy.
func dirtyCast(foos []Foo) []Bar {
	res := *(*[]Bar)(unsafe.Pointer(&foos))
	return res
}

type Foo struct {
	X int `json:"X"`
	Y int `json:"Y"`
}

type Bar struct {
	// swap X and Y :) plus produce different JSON
	Y int `json:"Y_flipped"`
	X int `json:"X_flipped"`
}


func main() {
	var foos []Foo = []Foo{
		{10, 20},
		{100, 200},
	}


	foosJson, _ := json.MarshalIndent(foos, "  ", "  ")
	fmt.Printf("foos JSON\n%s\n\n", foosJson)

	var bars []Bar = dirtyCast(foos)
	barsJson, _ := json.MarshalIndent(bars, "  ", "  ")
	fmt.Printf("bars JSON\n%s\n\n", barsJson)

	// Output:
	//	foos JSON
	//	[
	//		{
	//		  "X": 10,
	//		  "Y": 20
	//		},
	//		{
	//		  "X": 100,
	//		  "Y": 200
	//		}
	//	  ]
	//	
	//	bars JSON
	//	[
	//		{
	//		  "Y_flipped": 10,
	//		  "X_flipped": 20
	//		},
	//		{
	//		  "Y_flipped": 100,
	//		  "X_flipped": 200
	//		}
	//	  ]
}

trulyTerrifyingDirtyCast.go

package main

import (
	"encoding/json"
	"fmt"
	"reflect"
	"unsafe"
)

// dirtyCast converts from []Foo to []Bar without copy.
// Note that Bar has 2x number of fields. lol :)
func trulyTerrifyingDirtyCast(foos []Foo) []Bar {
	res := *(*[]Bar)(unsafe.Pointer(&foos))

	// adjust capacity and len since Bar has 2x number of fields
	header := (*reflect.SliceHeader)(unsafe.Pointer(&res))
	header.Cap = header.Cap / 2
	header.Len = header.Len / 2
	return res
}

type Foo struct {
	X int `json:"X"`
	Y int `json:"Y"`
}

type Bar struct {
	Y  int `json:"Y_flipped"`
	X  int `json:"X_flipped"`
	Y2 int `json:"Y2_flipped"`
	X2 int `json:"X2_flipped"`
}

func main() {
	var foos []Foo = []Foo{
		{10, 20},
		{100, 200},
	}

	foosJson, _ := json.MarshalIndent(foos, "  ", "  ")
	fmt.Printf("foos JSON\n  %s\n\n", foosJson)

	var bars []Bar = trulyTerrifyingDirtyCast(foos)
	barsJson, _ := json.MarshalIndent(bars, "  ", "  ")
	fmt.Printf("bars JSON\n  %s\n\n", barsJson)

	// Output:
	//	foos JSON
	//	  [
	//	    {
	//	      "X": 10,
	//	      "Y": 20
	//	    },
	//	    {
	//	      "X": 100,
	//	      "Y": 200
	//	    }
	//	  ]
	//	
	//	bars JSON
	//	  [
	//	    {
	//	      "Y_flipped": 10,
	//	      "X_flipped": 20,
	//	      "Y2_flipped": 100,
	//	      "X2_flipped": 200
	//	    }
	//	  ]
}