[core] Immutable Impls

2 files changed, 132 insertions, 0 deletions

diff --git a/include/mbgl/util/immutable.hpp b/include/mbgl/util/immutable.hpp
new file mode 100644
index 0000000000..1d6ff4079a
--- /dev/null
+++ b/include/mbgl/util/immutable.hpp
@@ -0,0 +1,127 @@
+#pragma once
+
+#include <memory>
+
+namespace mbgl {
+
+/**
+ * `Mutable<T>` is a non-nullable uniquely owning reference to a `T`. It can be efficiently converted
+ * to `Immutable<T>`.
+ * 
+ * The lifecycle of `Mutable<T>` and `Immutable<T>` is as follows:
+ * 
+ *   1. Create a `Mutable<T>` using `makeMutable(...)`
+ *   2. Mutate it freely
+ *   3. When you're ready to freeze its state and enable safe cross-thread sharing, move assign or
+ *      move construct it to `Immutable<T>`
+ *
+ * The reason that `Mutable<T>` exists, rather than simply using a `std::unique_ptr<T>`, is to take advantage
+ * of the underlying single-allocation optimization provided by `std::make_shared`.
+ */
+template <class T>
+class Mutable {
+public:
+    Mutable(Mutable&&) = default;
+    Mutable& operator=(Mutable&&) = default;
+
+    Mutable(const Mutable&) = delete;
+    Mutable& operator=(const Mutable&) = delete;
+
+    T* get() { return ptr.get(); }
+    T* operator->() { return ptr.get(); }
+    T& operator*() { return *ptr; }
+
+private:
+    Mutable(std::shared_ptr<T>&& s)
+        : ptr(std::move(s)) {}
+
+    std::shared_ptr<T> ptr;
+
+    template <class S> friend class Immutable;
+    template <class S, class... Args> friend Mutable<S> makeMutable(Args&&...);
+};
+
+template <class T, class... Args>
+Mutable<T> makeMutable(Args&&... args) {
+    return Mutable<T>(std::make_shared<T>(std::forward<Args>(args)...));
+}
+
+/**
+ * `Immutable<T>` is a non-nullable shared reference to a `const T`. Construction requires
+ * a transfer of unique ownership from a `Mutable<T>`; once constructed it has the same behavior
+ * as `std::shared_ptr<const T>` but with better indication of intent.
+ *
+ * Normally one should not share state between threads because it's difficult to verify the
+ * absence of read/write data races. `Immutable` provides a guarantee that no writes are
+ * possible, and instances therefore can be freely transferred and shared between threads.
+ */
+template <class T>
+class Immutable {
+public:
+    template <class S>
+    Immutable(Mutable<S>&& s)
+        : ptr(std::const_pointer_cast<const S>(std::move(s.ptr))) {}
+
+    template <class S>
+    Immutable(Immutable<S>&& s)
+        : ptr(std::move(s.ptr)) {}
+
+    template <class S>
+    Immutable(const Immutable<S>& s)
+        : ptr(s.ptr) {}
+
+    template <class S>
+    Immutable& operator=(Mutable<S>&& s) {
+        ptr = std::const_pointer_cast<const S>(std::move(s.ptr));
+        return *this;
+    }
+
+    template <class S>
+    Immutable& operator=(Immutable<S>&& s) {
+        ptr = std::move(s.ptr);
+        return *this;
+    }
+
+    template <class S>
+    Immutable& operator=(const Immutable<S>& s) {
+        ptr = s.ptr;
+        return *this;
+    }
+
+    const T* get() const { return ptr.get(); }
+    const T* operator->() const { return ptr.get(); }
+    const T& operator*() const { return *ptr; }
+
+    friend bool operator==(const Immutable<T>& lhs, const Immutable<T>& rhs) {
+        return lhs.ptr == rhs.ptr;
+    }
+
+    friend bool operator!=(const Immutable<T>& lhs, const Immutable<T>& rhs) {
+        return lhs.ptr != rhs.ptr;
+    }
+
+private:
+    Immutable(std::shared_ptr<const T>&& s)
+        : ptr(std::move(s)) {}
+
+    std::shared_ptr<const T> ptr;
+
+    template <class S> friend class Immutable;
+    template <class S> friend class EnableImmutableFromThis;
+    template <class S, class U> friend Immutable<S> staticImmutableCast(const Immutable<U>&);
+};
+
+template <class T>
+class EnableImmutableFromThis : public std::enable_shared_from_this<const T> {
+public:
+    Immutable<T> immutableFromThis() const {
+        return Immutable<T>(this->shared_from_this());
+    }
+};
+
+template <class S, class U>
+Immutable<S> staticImmutableCast(const Immutable<U>& u) {
+    return Immutable<S>(std::static_pointer_cast<const S>(u.ptr));
+}
+
+} // namespace mbgl
diff --git a/include/mbgl/util/tileset.hpp b/include/mbgl/util/tileset.hpp
index 1f28a5039a..1256e9fe96 100644
--- a/include/mbgl/util/tileset.hpp
+++ b/include/mbgl/util/tileset.hpp
@@ -18,6 +18,11 @@ public:
     Scheme scheme = Scheme::XYZ;
 
     // TileJSON also includes center, zoom, and bounds, but they are not used by mbgl.
+
+    friend bool operator==(const Tileset& lhs, const Tileset& rhs) {
+        return std::tie(lhs.tiles, lhs.zoomRange, lhs.attribution, lhs.scheme)
+            == std::tie(rhs.tiles, rhs.zoomRange, rhs.attribution, rhs.scheme);
+    }
 };
 
 } // namespace mbgl