Implement AssignOptionalField

blackmagic.go

@@ -5,6 +5,43 @@ import (
 	"reflect"
 )
 
+// AssignField is a convenience function to assign a value to
+// an optional struct field. In Go, an optional struct field is
+// usually denoted by a pointer to T instead of T:
+//
+//	type Object struct {
+//	  Optional *T
+//	}
+//
+// This gets a bit cumbersome when you want to assign literals
+// or you do not want to worry about taking the address of a
+// variable.
+//
+//	Object.Optional = &"foo" // doesn't compile!
+//
+// Instead you can use this function to do it in one line:
+//
+//	blackmagic.AssignOptionalField(&Object.Optionl, "foo")
+func AssignOptionalField(dst, src interface{}) error {
+	dstRV := reflect.ValueOf(dst)
+	srcRV := reflect.ValueOf(src)
+	if dstRV.Kind() != reflect.Pointer || dstRV.Elem().Kind() != reflect.Pointer {
+		return fmt.Errorf(`dst must be a pointer to a field that is turn a pointer of src (%T)`, src)
+	}
+
+	if !dstRV.Elem().CanSet() {
+		return fmt.Errorf(`dst (%T) is not assignable`, dstRV.Elem().Interface())
+	}
+	if !reflect.PtrTo(srcRV.Type()).AssignableTo(dstRV.Elem().Type()) {
+		return fmt.Errorf(`cannot assign src (%T) to dst (%T)`, src, dst)
+	}
+
+	ptr := reflect.New(srcRV.Type())
+	ptr.Elem().Set(srcRV)
+	dstRV.Elem().Set(ptr)
+	return nil
+}
+
 // AssignIfCompatible is a convenience function to safely
 // assign arbitrary values. dst must be a pointer to an
 // empty interface, or it must be a pointer to a compatible
@@ -26,7 +63,7 @@ func AssignIfCompatible(dst, src interface{}) error {
 
 	rv := reflect.ValueOf(dst)
 	if rv.Kind() != reflect.Ptr {
-		return fmt.Errorf(`argument to AssignIfCompatible() must be a pointer: %T`, dst)
+		return fmt.Errorf(`destination argument to AssignIfCompatible() must be a pointer: %T`, dst)
 	}
 
 	actualDst := rv.Elem()

blackmagic_test.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/lestrrat-go/blackmagic"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestAssignment(t *testing.T) {
@@ -59,3 +60,15 @@ func TestAssignment(t *testing.T) {
 		})
 	}
 }
+
+func TestAssignOptionalField(t *testing.T) {
+	var f struct {
+		Foo *string
+		Bar *int
+	}
+
+	require.NoError(t, blackmagic.AssignOptionalField(&f.Foo, "Hello"), `blackmagic.AssignOptionalField should succeed`)
+	require.Equal(t, *(f.Foo), "Hello")
+	require.NoError(t, blackmagic.AssignOptionalField(&f.Bar, 1), `blackmagic.AssignOptionalField should succeed`)
+	require.Equal(t, *(f.Bar), 1)
+}