1 files changed, 65 insertions, 0 deletions
diff --git a/third_party/nix/src/libutil/ref.hh b/third_party/nix/src/libutil/ref.hh
new file mode 100644
index 000000000000..063e6b327c22
--- /dev/null
+++ b/third_party/nix/src/libutil/ref.hh
@@ -0,0 +1,65 @@
+#pragma once
+
+#include <exception>
+#include <memory>
+#include <stdexcept>
+
+namespace nix {
+
+/* A simple non-nullable reference-counted pointer. Actually a wrapper
+   around std::shared_ptr that prevents non-null constructions. */
+template <typename T>
+class ref {
+ private:
+  std::shared_ptr<T> p;
+
+ public:
+  ref<T>(const ref<T>& r) : p(r.p) {}
+
+  explicit ref<T>(const std::shared_ptr<T>& p) : p(p) {
+    if (!p) {
+      throw std::invalid_argument("null pointer cast to ref");
+    }
+  }
+
+  explicit ref<T>(T* p) : p(p) {
+    if (!p) {
+      throw std::invalid_argument("null pointer cast to ref");
+    }
+  }
+
+  T* operator->() const { return &*p; }
+
+  T& operator*() const { return *p; }
+
+  operator std::shared_ptr<T>() const { return p; }
+
+  std::shared_ptr<T> get_ptr() const { return p; }
+
+  template <typename T2>
+  ref<T2> cast() const {
+    return ref<T2>(std::dynamic_pointer_cast<T2>(p));
+  }
+
+  template <typename T2>
+  std::shared_ptr<T2> dynamic_pointer_cast() const {
+    return std::dynamic_pointer_cast<T2>(p);
+  }
+
+  template <typename T2>
+  operator ref<T2>() const {
+    return ref<T2>((std::shared_ptr<T2>)p);
+  }
+
+ private:
+  template <typename T2, typename... Args>
+  friend ref<T2> make_ref(Args&&... args);
+};
+
+template <typename T, typename... Args>
+inline ref<T> make_ref(Args&&... args) {
+  auto p = std::make_shared<T>(std::forward<Args>(args)...);
+  return ref<T>(p);
+}
+
+}  // namespace nix