Allow missing and null pointer fields to be nil.

This commit ensures missing fields for pointer field types remain nil:

  * This allows one to distinguish the field wasn't present from the
    field being NULL when the PtrNil option is false.
  * This also makes it more convenient to reuse structs with pointer
    field types with various encoders like the standard library JSON
    marshaler, where you may want to omit a field if it didn't exist in
    the queried results.

The expectations around this change are captured in the
TestMissingFields test.

In addition, it also fixes an edge case where even with PtrNil set to
true, pointer field types would always return with a pointer to a zero
value instead of being nil even when the query property's value is
VT_NULL.

This fix is checked by the TestNullPointerField test.

swbemservices.go

@@ -1,3 +1,4 @@
+//go:build windows
 // +build windows
 
 package wmi

swbemservices_test.go

@@ -1,3 +1,4 @@
+//go:build windows
 // +build windows
 
 package wmi
@@ -103,8 +104,8 @@ func WbemGetMemoryUsageMB(s *SWbemServices) (float64, float64, float64) {
 //Run all benchmarks (should run for at least 60s to get a stable number):
 //go test -run=NONE -bench=Version -benchtime=120s
 
-//Individual benchmarks:
-//go test -run=NONE -bench=NewVersion -benchtime=120s
+// Individual benchmarks:
+// go test -run=NONE -bench=NewVersion -benchtime=120s
 func BenchmarkNewVersion(b *testing.B) {
 	s, err := InitializeSWbemServices(DefaultClient)
 	if err != nil {
@@ -128,7 +129,7 @@ func BenchmarkNewVersion(b *testing.B) {
 	}
 }
 
-//go test -run=NONE -bench=OldVersion -benchtime=120s
+// go test -run=NONE -bench=OldVersion -benchtime=120s
 func BenchmarkOldVersion(b *testing.B) {
 	var dst []Win32_OperatingSystem
 	q := CreateQuery(&dst, "")

wmi.go

@@ -1,3 +1,4 @@
+//go:build windows
 // +build windows
 
 /*
@@ -20,7 +21,6 @@ Example code to print names of running processes:
 			println(i, v.Name)
 		}
 	}
-
 */
 package wmi
 
@@ -338,11 +338,6 @@ func (c *Client) loadEntity(dst interface{}, src *ole.IDispatch) (errFieldMismat
 		f := v.Field(i)
 		of := f
 		isPtr := f.Kind() == reflect.Ptr
-		if isPtr {
-			ptr := reflect.New(f.Type().Elem())
-			f.Set(ptr)
-			f = f.Elem()
-		}
 		n := v.Type().Field(i).Name
 		if n[0] < 'A' || n[0] > 'Z' {
 			continue
@@ -367,6 +362,12 @@ func (c *Client) loadEntity(dst interface{}, src *ole.IDispatch) (errFieldMismat
 		}
 		defer prop.Clear()
 
+		if isPtr && !(c.PtrNil && prop.VT == 0x1) {
+			ptr := reflect.New(f.Type().Elem())
+			f.Set(ptr)
+			f = f.Elem()
+		}
+
 		if prop.VT == 0x1 { //VT_NULL
 			continue
 		}

wmi_test.go

@@ -1,3 +1,4 @@
+//go:build windows
 // +build windows
 
 package wmi
@@ -39,6 +40,87 @@ func TestFieldMismatch(t *testing.T) {
 	}
 }
 
+func TestMissingFields(t *testing.T) {
+	type s struct {
+		Name           string
+		Missing        uint32
+		MissingPointer *uint32
+	}
+
+	var dst []s
+
+	client := &Client{
+		AllowMissingFields: true,
+	}
+	err := client.Query("SELECT Name FROM Win32_Process", &dst)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i := range dst {
+		if dst[i].Missing != 0 {
+			t.Fatal("Expected Missing field to be 0")
+		}
+		if dst[i].MissingPointer != nil {
+			t.Fatal("Expected MissingPointer field to be nil")
+		}
+	}
+
+	// NonePtrZero and PtrNil should only affect the behavior of fields that
+	// exist as result properties, not missing fields.
+	client = &Client{
+		NonePtrZero:        true,
+		PtrNil:             true,
+		AllowMissingFields: true,
+	}
+	dst = []s{}
+	err = client.Query("SELECT Name FROM Win32_Process", &dst)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i := range dst {
+		if dst[i].Missing != 0 {
+			t.Fatal("Expected Missing field to be 0")
+		}
+		if dst[i].MissingPointer != nil {
+			t.Fatal("Expected MissingPointer field to be nil")
+		}
+	}
+}
+
+func TestNullPointerField(t *testing.T) {
+	type s struct {
+		Name   string
+		Status *string
+	}
+
+	var dst []s
+
+	client := &Client{}
+	err := client.Query("SELECT Name, Status FROM Win32_Process WHERE Status IS NULL", &dst)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i := range dst {
+		if dst[i].Status == nil {
+			t.Fatal("Expected Status field to not be nil")
+		}
+	}
+
+	client = &Client{
+		PtrNil: true,
+	}
+	dst = []s{}
+	err = client.Query("SELECT Name, Status FROM Win32_Process WHERE Status IS NULL", &dst)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i := range dst {
+		if dst[i].Status != nil {
+			t.Fatal("Expected Status field to be nil")
+		}
+	}
+}
+
 func TestStrings(t *testing.T) {
 	printed := false
 	f := func() {