From 58bb86fd2f0ecd740fc5bc8078de44221c12c70a Mon Sep 17 00:00:00 2001
From: Steven Le Rouzic <steven.lerouzic@gmail.com>
Date: Fri, 22 Nov 2024 17:13:03 +0100
Subject: Add a niche to box

---
 asl/annotations.hpp     |  1 -
 asl/box.hpp             | 16 ++++++++++++----
 asl/meta.hpp            |  9 +++++++++
 asl/tests/box_tests.cpp | 22 ++++++----------------
 4 files changed, 27 insertions(+), 21 deletions(-)

(limited to 'asl')

diff --git a/asl/annotations.hpp b/asl/annotations.hpp
index a398e13..fc65378 100644
--- a/asl/annotations.hpp
+++ b/asl/annotations.hpp
@@ -2,7 +2,6 @@
 
 #include "asl/config.hpp"
 
-
 #if ASL_COMPILER_CLANG_CL
     #define ASL_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
 #elif ASL_COMPILER_CLANG
diff --git a/asl/box.hpp b/asl/box.hpp
index d8ccd36..7556d76 100644
--- a/asl/box.hpp
+++ b/asl/box.hpp
@@ -16,15 +16,18 @@ class box
     ASL_NO_UNIQUE_ADDRESS Allocator m_alloc;
     
 public:
-    explicit constexpr box(T* ptr = nullptr)
+    explicit constexpr box(niche)
         requires default_constructible<Allocator>
-        : m_ptr{ptr}
+        : m_ptr{nullptr}
+        , m_alloc{}
     {}
-    
+
     constexpr box(T* ptr, Allocator alloc)
         : m_ptr{ptr}
         , m_alloc{ASL_MOVE(alloc)}
-    {}
+    {
+        ASL_ASSERT(m_ptr != nullptr);
+    }
 
     constexpr box(box&& other)
         : m_ptr{exchange(other.m_ptr, nullptr)}
@@ -77,6 +80,11 @@ public:
         ASL_ASSERT(m_ptr != nullptr);
         return m_ptr;
     }
+
+    constexpr bool operator==(niche) const
+    {
+        return m_ptr == nullptr;
+    }
 };
 
 template<is_object T, allocator Allocator = DefaultAllocator, typename... Args>
diff --git a/asl/meta.hpp b/asl/meta.hpp
index f0397a1..72b36fd 100644
--- a/asl/meta.hpp
+++ b/asl/meta.hpp
@@ -160,4 +160,13 @@ template<typename T, int N> struct _is_array_helper<T[N]> : true_type  {};
 
 template<typename T> concept is_array = _is_array_helper<T>::value;
 
+struct niche {};
+
+template<typename T>
+concept has_niche = constructible_from<T, niche> &&
+    requires (const T& value, niche n)
+    {
+        { value == n } -> same_as<bool>;
+    };
+
 } // namespace asl
diff --git a/asl/tests/box_tests.cpp b/asl/tests/box_tests.cpp
index f1a35f3..3813639 100644
--- a/asl/tests/box_tests.cpp
+++ b/asl/tests/box_tests.cpp
@@ -4,30 +4,20 @@
 #include "asl/tests/test_types.hpp"
 
 static_assert(sizeof(asl::box<int>) == sizeof(int*));
-static_assert(asl::default_constructible<asl::box<int>>);
 static_assert(!asl::copyable<asl::box<int>>);
 static_assert(asl::moveable<asl::box<int>>);
-static_assert(asl::default_constructible<asl::box<NonMoveConstructible>>);
+static_assert(asl::has_niche<asl::box<int>>);
 
 ASL_TEST(destructor)
 {
     bool d = false;
 
     {
-        asl::box<DestructorObserver> box2;
-    
-        {
-            auto box = asl::make_box<DestructorObserver>(&d);
-            ASL_TEST_ASSERT(!d);
-    
-
-            auto box3 = ASL_MOVE(box);
-            ASL_TEST_ASSERT(!d);
-
-            box2 = ASL_MOVE(box3);
-            ASL_TEST_ASSERT(!d);
-        }
-    
+        auto box = asl::make_box<DestructorObserver>(&d);
+        ASL_TEST_ASSERT(!d);
+
+
+        auto box3 = ASL_MOVE(box);
         ASL_TEST_ASSERT(!d);
     }
 
-- 
cgit