tensor/Drop: updated comment

example/basic/main.go

@@ -1,7 +1,7 @@
 package main
 
 import (
-	// "fmt"
+	"fmt"
 
 	"github.com/sugarme/gotch"
 	ts "github.com/sugarme/gotch/tensor"
@@ -13,4 +13,10 @@ func main() {
 	tensor := ts.MustArange(ts.IntScalar(2*3*4), gotch.Int64, gotch.CPU).MustView([]int64{2, 3, 4}, true)
 
 	tensor.Print()
+
+	fmt.Printf("tensor is nil: %v\n", tensor.IsNil())
+
+	tensor.MustDrop()
+
+	fmt.Printf("tensor is nil: %v\n", tensor.IsNil())
 }

tensor/tensor.go

@@ -874,6 +874,7 @@ func (ts Tensor) MustToString(lw int64) (retVal string) {
 // Drop drops (frees) the tensor
 func (ts Tensor) Drop() (err error) {
 
+	// has not freed yet.
 	if !ts.MustDefined() {
 		return nil
 	}
@@ -883,11 +884,13 @@ func (ts Tensor) Drop() (err error) {
 		return err
 	}
 
-	// NOTE. assign to a new undefined tensor, then check `ts.MustDefined`
-	// before deleting at C land. Hence `Drop` method can be called
-	// multiple times without worrying about double C memory delete panic.
-	// Other pattern is `defer ts.MustDrop()` whenever a tensor is created.
-	ts = NewTensor()
+	// NOTE. there is no reliable way to tell if a pointer has been freed.
+	// Ref. https://stackoverflow.com/questions/8300853
+	// This is a hacky way: as soon as tensor is free up, turn it into "undefined" tensor.
+	// So that next time, call `ts.MustDefined()` to check whether tensor has been freed.
+	// This is useful as when can call `ts.MustDrop()` multiple times without worrying
+	// about "free(): double free detected..." error.
+	ts = NewTensor() // ts now is "undefined"
 
 	return nil
 }
@@ -1165,3 +1168,10 @@ func (ts Tensor) Swish() (retVal Tensor) {
 func (ts Tensor) AvgPool2DDefault(ksize int64, del bool) (retVal Tensor) {
 	return ts.MustAvgPool2d([]int64{ksize, ksize}, []int64{ksize, ksize}, []int64{0, 0}, false, true, 1, del)
 }
+
+func (ts Tensor) IsNil() bool {
+
+	C.free(unsafe.Pointer(ts.ctensor))
+
+	return unsafe.Pointer(ts.ctensor) == nil
+}