sparrow::nullable implementation

CMakeLists.txt

@@ -107,6 +107,7 @@ set(SPARROW_HEADERS
     ${SPARROW_INCLUDE_DIR}/sparrow/memory.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/mp_utils.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/null_layout.hpp
+    ${SPARROW_INCLUDE_DIR}/sparrow/nullable.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/sparrow_version.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/typed_array.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/variable_size_binary_layout.hpp

include/sparrow/mp_utils.hpp

@@ -1,4 +1,4 @@
 // Copyright 2024 Man Group Operations Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -355,6 +355,15 @@
     //////////////////////////////////////////////////
     //// Miscellaneous ///////////////////////////////
 
+    template <class T>
+    struct add_const_lvalue_reference
+        : std::add_lvalue_reference<std::add_const_t<T>>
+    {
+    };
+
+    template <class T>
+    using add_const_lvalue_reference_t = typename add_const_lvalue_reference<T>::type;
+
     template <class T, bool is_const>
     struct constify : std::conditional<is_const, const T, T>
     {
@@ -410,4 +419,9 @@
 #endif
     }
 
+    /// Matches types that can be convertible to and assignable from bool. We do not use
+    /// `std::convertible_to` because we don't want to impose an implicit conversion.
+    template <class T>
+    concept boolean_like = std::is_assignable_v<std::add_lvalue_reference_t<T>, bool> and 
+                           requires { static_cast<bool>(std::declval<T>()); };
 }