lottie/vector: added cow_ptr class to support copy_on_write class in vector.

Change-Id: I485637da44d44ce473dc760aaf484b0377bdcda1

src/vector/vcowptr.h

@@ -0,0 +1,82 @@
+#ifndef VCOWPTR_H
+#define VCOWPTR_H
+
+#include<assert.h>
+
+template <typename T>
+class vcow_ptr {
+    struct model {
+        std::atomic<std::size_t> mRef{1};
+
+        model() = default;
+
+        template <class... Args>
+        explicit model(Args&&... args) : mValue(std::forward<Args>(args)...) {}
+
+        T mValue;
+    };
+    model* mModel;
+public:
+    using element_type = T;
+
+    vcow_ptr() {
+        static model default_s;
+        mModel = &default_s;
+        ++mModel->mRef;
+    }
+
+    ~vcow_ptr() {
+        if (mModel && (--mModel->mRef == 0)) delete mModel;
+    }
+
+    template <class... Args>
+    vcow_ptr(Args&&... args)
+        : mModel(new model(std::forward<Args>(args)...)) {}
+
+    vcow_ptr(const vcow_ptr& x) noexcept : mModel(x.mModel) {
+        assert(mModel);
+        ++mModel->mRef;
+    }
+    vcow_ptr(vcow_ptr&& x) noexcept : mModel(x.mModel) {
+        assert(mModel);
+        x.mModel = nullptr;
+    }
+
+    auto operator=(const vcow_ptr& x) noexcept -> vcow_ptr& {
+        return *this = vcow_ptr(x);
+    }
+
+    auto operator=(vcow_ptr&& x) noexcept -> vcow_ptr& {
+        auto tmp = std::move(x);
+        swap(*this, tmp);
+        return *this;
+    }
+
+    auto operator*() const noexcept -> const element_type& { return read(); }
+
+    auto operator-> () const noexcept -> const element_type* { return &read(); }
+
+    bool unique() const noexcept {
+        assert(mModel);
+
+        return mModel->mRef == 1;
+    }
+
+    auto write() -> element_type& {
+        if (!unique()) *this = vcow_ptr(read());
+
+        return mModel->mValue;
+    }
+
+    auto read() const noexcept -> const element_type& {
+        assert(mModel);
+
+        return mModel->mValue;
+    }
+
+    friend inline void swap(vcow_ptr& x, vcow_ptr& y) noexcept {
+        std::swap(x.mModel, y.mModel);
+    }
+};
+
+#endif // VCOWPTR_H