1 files changed, 121 insertions, 0 deletions
diff --git a/src/libutil/box_ptr.hh b/src/libutil/box_ptr.hh
new file mode 100644
index 000000000..f7e4fd509
--- /dev/null
+++ b/src/libutil/box_ptr.hh
@@ -0,0 +1,121 @@
+#pragma once
+/// @file
+
+#include <concepts>
+#include <memory>
+#include <stdexcept>
+#include <type_traits>
+#include <assert.h>
+
+namespace nix {
+
+/** A pointer that's like Rust's Box: forwards comparisons to the inner class and is non-null */
+template<typename T>
+// FIXME: add custom deleter support
+class box_ptr
+{
+    std::unique_ptr<T> inner;
+
+    template<typename T2>
+    friend class box_ptr;
+
+    explicit box_ptr(std::unique_ptr<T> p)
+        : inner(std::move(p))
+    {
+        assert(inner != nullptr);
+    }
+
+public:
+    using pointer = typename std::unique_ptr<T>::pointer;
+
+    inline typename std::add_lvalue_reference<T>::type operator*() const noexcept
+    {
+        return *inner.get();
+    }
+
+    inline pointer operator->() const noexcept
+    {
+        return inner.get();
+    }
+
+    inline pointer get() const noexcept
+    {
+        return inner.get();
+    }
+
+    /**
+     * Create a box_ptr from a nonnull unique_ptr.
+     */
+    static inline box_ptr<T> unsafeFromNonnull(std::unique_ptr<T> p)
+    {
+        return box_ptr(std::move(p));
+    }
+
+    inline box_ptr<T> & operator=(box_ptr<T> && other) noexcept = default;
+
+    // No copy for you.
+    box_ptr<T> & operator=(const box_ptr<T> &) = delete;
+
+    // XXX: we have to set the other's insides to nullptr, since we cannot
+    // put a garbage object there, and we don't have any compiler
+    // enforcement to not touch moved-from values. sighh.
+    box_ptr(box_ptr<T> && other) = default;
+
+    /** Conversion operator */
+    template<typename Other>
+    // n.b. the requirements here are the same as libstdc++ unique_ptr's checks but with concepts
+    requires std::convertible_to<typename box_ptr<Other>::pointer, pointer> &&(!std::is_array_v<Other>)
+        box_ptr(box_ptr<Other> && other) noexcept
+        : inner(std::move(other.inner))
+    {
+        other.inner = nullptr;
+    }
+
+    box_ptr(box_ptr<T> & other) = delete;
+};
+
+template<typename T>
+requires std::equality_comparable<T>
+bool operator==(box_ptr<T> const & x, box_ptr<T> const & y)
+{
+    // Although there could be an optimization here that compares x == y, this
+    // is unsound for floats with NaN, or anything else that violates
+    // reflexivity.
+    return *x == *y;
+}
+
+template<typename T>
+requires std::equality_comparable<T>
+bool operator!=(box_ptr<T> const & x, box_ptr<T> const & y)
+{
+    return *x != *y;
+}
+
+#define MAKE_COMPARISON(OP) \
+    template<typename T> \
+    requires std::totally_ordered<T> \
+    bool operator OP(box_ptr<T> const & x, box_ptr<T> const & y) \
+    { \
+        return *x OP * y; \
+    }
+
+MAKE_COMPARISON(<);
+MAKE_COMPARISON(>);
+MAKE_COMPARISON(>=);
+MAKE_COMPARISON(<=);
+
+#undef MAKE_COMPARISON
+
+template<typename T>
+requires std::three_way_comparable<T> std::compare_three_way_result_t<T, T>
+operator<=>(box_ptr<T> const & x, box_ptr<T> const & y)
+{
+    return *x <=> *y;
+}
+
+template<typename T, typename... Args>
+inline box_ptr<T> make_box_ptr(Args &&... args)
+{
+    return box_ptr<T>::unsafeFromNonnull(std::make_unique<T>(std::forward<Args>(args)...));
+}
+};