The test code below demonstrates a json.Unmarshal behavior that I find
unexpected. The code unmarshals arrays of objects with pointers fields. All
unmarshaled objects end up sharing the same pointer value, effectively
overwriting the contents of the pointer.
package unmarshall
import (
"encoding/json"
"fmt"
"testing"
"github.com/stretchr/testify/assert"
)
func TestUnmarshal(t *testing.T) {
type Elem struct {
V int
P *int
}
elems := []Elem(nil)
newElems := []Elem(nil)
json.Unmarshal([]byte(`[{"V":10, "P":1}]`), &newElems)
elems = append(elems, newElems...)
json.Unmarshal([]byte(`[{"V":20, "P":2}]`), &newElems)
elems = append(elems, newElems...)
assert.Equal(t, 10, elems[0].V)
assert.Equal(t, 1, *elems[0].P)
assert.Equal(t, 20, elems[1].V)
assert.Equal(t, 2, *elems[1].P)
assert.NotEqual(t, elems[0].P, elems[1].P)
}
I find the behavior unexpected because the documentation of json.Unmarshal
states:
// To unmarshal a JSON array into a slice, Unmarshal resets the slice length
// to zero and then appends each element to the slice.
However, reseting the slice length to zero and appending elements to the
slice results in different behavior:
func TestAppend(t *testing.T) {
type Elem struct {
V int
P *int
}
elems := []Elem(nil)
newElems := []Elem(nil)
one, two := 1, 2
newElems = append(newElems[:0], Elem{V: 10, P: &one})
elems = append(elems, newElems...)
newElems = append(newElems[:0], Elem{V: 20, P: &two})
elems = append(elems, newElems...)
assert.Equal(t, 10, elems[0].V)
assert.Equal(t, 1, *elems[0].P)
assert.Equal(t, 20, elems[1].V)
assert.Equal(t, 2, *elems[1].P)
assert.NotEqual(t, elems[0].P, elems[1].P)
}
Are my expectations of the above behavior incorrect?
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