diff --git a/blob.go b/blob.go
index 4cee876..4277127 100644
--- a/blob.go
+++ b/blob.go
@@ -20,15 +20,16 @@ import (
 
 type Blob struct {
 	gitObject
+	cast_ptr *C.git_blob
 }
 
 func (v *Blob) Size() int64 {
-	return int64(C.git_blob_rawsize(v.ptr))
+	return int64(C.git_blob_rawsize(v.cast_ptr))
 }
 
 func (v *Blob) Contents() []byte {
-	size := C.int(C.git_blob_rawsize(v.ptr))
-	buffer := unsafe.Pointer(C.git_blob_rawcontent(v.ptr))
+	size := C.int(C.git_blob_rawsize(v.cast_ptr))
+	buffer := unsafe.Pointer(C.git_blob_rawcontent(v.cast_ptr))
 	return C.GoBytes(buffer, size)
 }
 
diff --git a/branch.go b/branch.go
index aee23e4..bd7312b 100644
--- a/branch.go
+++ b/branch.go
@@ -43,7 +43,7 @@ func (repo *Repository) CreateBranch(branchName string, target *Commit, force bo
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	ret := C.git_branch_create(&ref.ptr, repo.ptr, cBranchName, target.ptr, cForce, cSignature, cmsg)
+	ret := C.git_branch_create(&ref.ptr, repo.ptr, cBranchName, target.cast_ptr, cForce, cSignature, cmsg)
 	if ret < 0 {
 		return nil, MakeGitError(ret)
 	}
diff --git a/commit.go b/commit.go
index 0edebb6..0a5cfce 100644
--- a/commit.go
+++ b/commit.go
@@ -17,56 +17,57 @@ import (
 // Commit
 type Commit struct {
 	gitObject
+	cast_ptr *C.git_commit
 }
 
 func (c Commit) Message() string {
-	return C.GoString(C.git_commit_message(c.ptr))
+	return C.GoString(C.git_commit_message(c.cast_ptr))
 }
 
 func (c Commit) Tree() (*Tree, error) {
-	var ptr *C.git_object
+	var ptr *C.git_tree
 
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	err := C.git_commit_tree(&ptr, c.ptr)
+	err := C.git_commit_tree(&ptr, c.cast_ptr)
 	if err < 0 {
 		return nil, MakeGitError(err)
 	}
 
-	return allocObject(ptr).(*Tree), nil
+	return allocObject((*C.git_object)(ptr)).(*Tree), nil
 }
 
 func (c Commit) TreeId() *Oid {
-	return newOidFromC(C.git_commit_tree_id(c.ptr))
+	return newOidFromC(C.git_commit_tree_id(c.cast_ptr))
 }
 
 func (c Commit) Author() *Signature {
-	ptr := C.git_commit_author(c.ptr)
-	return newSignatureFromC(ptr)
+	cast_ptr := C.git_commit_author(c.cast_ptr)
+	return newSignatureFromC(cast_ptr)
 }
 
 func (c Commit) Committer() *Signature {
-	ptr := C.git_commit_committer(c.ptr)
-	return newSignatureFromC(ptr)
+	cast_ptr := C.git_commit_committer(c.cast_ptr)
+	return newSignatureFromC(cast_ptr)
 }
 
 func (c *Commit) Parent(n uint) *Commit {
-	var cobj *C.git_object
-	ret := C.git_commit_parent(&cobj, c.ptr, C.uint(n))
+	var cobj *C.git_commit
+	ret := C.git_commit_parent(&cobj, c.cast_ptr, C.uint(n))
 	if ret != 0 {
 		return nil
 	}
 
-	return allocObject(cobj).(*Commit)
+	return allocObject((*C.git_object)(cobj)).(*Commit)
 }
 
 func (c *Commit) ParentId(n uint) *Oid {
-	return newOidFromC(C.git_commit_parent_id(c.ptr, C.uint(n)))
+	return newOidFromC(C.git_commit_parent_id(c.cast_ptr, C.uint(n)))
 }
 
 func (c *Commit) ParentCount() uint {
-	return uint(C.git_commit_parentcount(c.ptr))
+	return uint(C.git_commit_parentcount(c.cast_ptr))
 }
 
 // Signature
diff --git a/object.go b/object.go
index 090be1f..101d15e 100644
--- a/object.go
+++ b/object.go
@@ -48,7 +48,7 @@ func (t ObjectType) String() (string) {
 }
 
 func (o gitObject) Id() *Oid {
-	return newOidFromC(C.git_commit_id(o.ptr))
+	return newOidFromC(C.git_object_id(o.ptr))
 }
 
 func (o gitObject) Type() ObjectType {
@@ -57,24 +57,33 @@ func (o gitObject) Type() ObjectType {
 
 func (o *gitObject) Free() {
 	runtime.SetFinalizer(o, nil)
-	C.git_commit_free(o.ptr)
+	C.git_object_free(o.ptr)
 }
 
 func allocObject(cobj *C.git_object) Object {
 
 	switch ObjectType(C.git_object_type(cobj)) {
 	case ObjectCommit:
-		commit := &Commit{gitObject{cobj}}
+		commit := &Commit{
+			gitObject: gitObject{cobj},
+			cast_ptr:  (*C.git_commit)(cobj),
+		}
 		runtime.SetFinalizer(commit, (*Commit).Free)
 		return commit
 
 	case ObjectTree:
-		tree := &Tree{gitObject{cobj}}
+		tree := &Tree{
+			gitObject: gitObject{cobj},
+			cast_ptr:  (*C.git_tree)(cobj),
+		}
 		runtime.SetFinalizer(tree, (*Tree).Free)
 		return tree
 
 	case ObjectBlob:
-		blob := &Blob{gitObject{cobj}}
+		blob := &Blob{
+			gitObject: gitObject{cobj},
+			cast_ptr:  (*C.git_blob)(cobj),
+		}
 		runtime.SetFinalizer(blob, (*Blob).Free)
 		return blob
 	}
diff --git a/repository.go b/repository.go
index d6eadc8..d757747 100644
--- a/repository.go
+++ b/repository.go
@@ -239,7 +239,7 @@ func (v *Repository) CreateCommit(
 	if nparents > 0 {
 		cparents = make([]*C.git_commit, nparents)
 		for i, v := range parents {
-			cparents[i] = v.ptr
+			cparents[i] = v.cast_ptr
 		}
 		parentsarg = &cparents[0]
 	}
@@ -256,7 +256,7 @@ func (v *Repository) CreateCommit(
 	ret := C.git_commit_create(
 		oid.toC(), v.ptr, cref,
 		authorSig, committerSig,
-		nil, cmsg, tree.ptr, C.size_t(nparents), parentsarg)
+		nil, cmsg, tree.cast_ptr, C.size_t(nparents), parentsarg)
 
 	if ret < 0 {
 		return nil, MakeGitError(ret)
@@ -331,7 +331,7 @@ func (v *Repository) TreeBuilderFromTree(tree *Tree) (*TreeBuilder, error) {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	if ret := C.git_treebuilder_create(&bld.ptr, tree.ptr); ret < 0 {
+	if ret := C.git_treebuilder_create(&bld.ptr, tree.cast_ptr); ret < 0 {
 		return nil, MakeGitError(ret)
 	}
 	runtime.SetFinalizer(bld, (*TreeBuilder).Free)
diff --git a/tree.go b/tree.go
index 7070ac7..8356fba 100644
--- a/tree.go
+++ b/tree.go
@@ -26,6 +26,7 @@ const (
 
 type Tree struct {
 	gitObject
+	cast_ptr *C.git_tree
 }
 
 type TreeEntry struct {
@@ -48,7 +49,7 @@ func (t Tree) EntryByName(filename string) *TreeEntry {
 	cname := C.CString(filename)
 	defer C.free(unsafe.Pointer(cname))
 
-	entry := C.git_tree_entry_byname(t.ptr, cname)
+	entry := C.git_tree_entry_byname(t.cast_ptr, cname)
 	if entry == nil {
 		return nil
 	}
@@ -66,7 +67,7 @@ func (t Tree) EntryByPath(path string) (*TreeEntry, error) {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	ret := C.git_tree_entry_bypath(&entry, t.ptr, cpath)
+	ret := C.git_tree_entry_bypath(&entry, t.cast_ptr, cpath)
 	if ret < 0 {
 		return nil, MakeGitError(ret)
 	}
@@ -75,7 +76,7 @@ func (t Tree) EntryByPath(path string) (*TreeEntry, error) {
 }
 
 func (t Tree) EntryByIndex(index uint64) *TreeEntry {
-	entry := C.git_tree_entry_byindex(t.ptr, C.size_t(index))
+	entry := C.git_tree_entry_byindex(t.cast_ptr, C.size_t(index))
 	if entry == nil {
 		return nil
 	}
@@ -84,7 +85,7 @@ func (t Tree) EntryByIndex(index uint64) *TreeEntry {
 }
 
 func (t Tree) EntryCount() uint64 {
-	num := C.git_tree_entrycount(t.ptr)
+	num := C.git_tree_entrycount(t.cast_ptr)
 	return uint64(num)
 }
 
@@ -104,7 +105,7 @@ func (t Tree) Walk(callback TreeWalkCallback) error {
 	defer runtime.UnlockOSThread()
 
 	err := C._go_git_treewalk(
-		t.ptr,
+		t.cast_ptr,
 		C.GIT_TREEWALK_PRE,
 		unsafe.Pointer(&callback),
 	)