From 3b0afae3d132df16b82ffde72631289c36ef3a36 Mon Sep 17 00:00:00 2001
From: Yang Yu <yuyang18@baidu.com>
Date: Mon, 8 Jan 2018 14:21:45 +0800
Subject: [PATCH] Add more comments

---
 paddle/framework/details/cow_ptr.h       | 34 +++++++++++++-----------
 paddle/framework/details/cow_ptr_test.cc |  4 ---
 2 files changed, 19 insertions(+), 19 deletions(-)

diff --git a/paddle/framework/details/cow_ptr.h b/paddle/framework/details/cow_ptr.h
index 6f1dcab40..7e308ffb5 100644
--- a/paddle/framework/details/cow_ptr.h
+++ b/paddle/framework/details/cow_ptr.h
@@ -25,13 +25,14 @@ class ThreadUnsafeOwnershipFlags {
  public:
   ThreadUnsafeOwnershipFlags(bool flag) : flag_(flag) {}
 
-  ThreadUnsafeOwnershipFlags(const ThreadUnsafeOwnershipFlags& o) = delete;
-  ThreadUnsafeOwnershipFlags& operator=(const ThreadUnsafeOwnershipFlags& o) =
-      delete;
-  ThreadUnsafeOwnershipFlags(ThreadUnsafeOwnershipFlags&& o) = default;
+  ThreadUnsafeOwnershipFlags(const ThreadUnsafeOwnershipFlags& other) = delete;
+  ThreadUnsafeOwnershipFlags& operator=(
+      const ThreadUnsafeOwnershipFlags& other) = delete;
+  ThreadUnsafeOwnershipFlags(ThreadUnsafeOwnershipFlags&& other) = default;
 
   void SetOwnership(bool flag) { flag_ = flag; }
 
+  // Invoke the callback if it is not owned.
   template <typename Callback>
   void AcquireOwnershipOnce(Callback acquire) {
     if (!flag_) {
@@ -44,7 +45,7 @@ class ThreadUnsafeOwnershipFlags {
   bool flag_;
 };
 
-// Copy On Write pointer.
+// Copy-On-Write pointer.
 // It will hold a T* pointer, and only copy once when `MutableData` is invoked.
 //
 // The template parameter OwnershipFlags should have:
@@ -52,6 +53,8 @@ class ThreadUnsafeOwnershipFlags {
 //   * SetOwnership(bool flag).
 //   * AcquireOwnershipOnce(Callback). It will invoke the callback if it is not
 //     owned.
+//
+// https://en.wikipedia.org/wiki/Copy-on-write
 template <typename T, typename OwnershipFlags = ThreadUnsafeOwnershipFlags>
 class COWPtr {
  public:
@@ -59,33 +62,34 @@ class COWPtr {
   explicit COWPtr(T* ptr) : payload_(ptr), ownership_{true} {}
 
   // Move methods. Steal ownership from origin
-  COWPtr(COWPtr&& o)
-      : payload_(o.payload_), ownership_{std::move(o.ownership_)} {}
+  COWPtr(COWPtr&& other)
+      : payload_(other.payload_), ownership_{std::move(other.ownership_)} {}
   COWPtr& operator=(COWPtr&& origin) = default;
 
   // Copy methods. Not own payload
-  COWPtr(const COWPtr& o) : payload_(o.payload_), ownership_{false} {}
-  COWPtr& operator=(const COWPtr& o) {
-    payload_ = o.payload_;
+  COWPtr(const COWPtr& other) : payload_(other.payload_), ownership_{false} {}
+  COWPtr& operator=(const COWPtr& other) {
+    payload_ = other.payload_;
     ownership_.SetOwnership(false);
     return *this;
   }
 
+  // Access read only data.
   const T& Data() const { return *payload_; }
 
+  // Access mutable data. If the data is not owned, the data will be copied
+  // before.
   T* MutableData() {
     ownership_.AcquireOwnershipOnce(
         [this] { payload_.reset(new T(*payload_)); });
     return payload_.get();
   }
 
-  void Reset() {
-    ownership_.AcquireOwnershipOnce([this] { payload_.reset(); });
-    payload_.reset(new T());
-  }
-
  private:
+  // Actual data pointer.
   std::shared_ptr<T> payload_;
+
+  // Ownership flag.
   OwnershipFlags ownership_;
 };
 
diff --git a/paddle/framework/details/cow_ptr_test.cc b/paddle/framework/details/cow_ptr_test.cc
index 080a0a0a4..936954a23 100644
--- a/paddle/framework/details/cow_ptr_test.cc
+++ b/paddle/framework/details/cow_ptr_test.cc
@@ -28,10 +28,6 @@ TEST(COWPtr, all) {
   *ptr2.MutableData() = 10;
   ASSERT_EQ(ptr.Data(), 0);
   ASSERT_EQ(ptr2.Data(), 10);
-
-  auto ptr_before = ptr2.MutableData();
-  ptr2.Reset();
-  ASSERT_NE(ptr2.MutableData(), ptr_before);
 }
 
 }  // namespace details
-- 
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