Rewrite the CFOA containers' FOA constructors so that a class with a template conversion operator does not instantiate the FOA container

Author: k3DW

include/boost/unordered/concurrent_flat_map.hpp

@@ -2,6 +2,7 @@
  *
  * Copyright 2023 Christian Mazakas.
  * Copyright 2023-2026 Joaquin M Lopez Munoz.
+ * Copyright 2026 Braden Ganetsky
  * Distributed under the Boost Software License, Version 1.0.
  * (See accompanying file LICENSE_1_0.txt or copy at
  * http://www.boost.org/LICENSE_1_0.txt)
@@ -189,10 +190,10 @@ namespace boost {
       {
       }
 
-
-      template <bool avoid_explicit_instantiation = true>
+      template <typename Key2,
+        typename std::enable_if<std::is_same<Key, Key2>::value, int>::type = 0>
       concurrent_flat_map(
-        unordered_flat_map<Key, T, Hash, Pred, Allocator>&& other)
+        unordered_flat_map<Key2, T, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))
       {
       }

include/boost/unordered/concurrent_flat_set.hpp

@@ -2,6 +2,7 @@
  *
  * Copyright 2023 Christian Mazakas.
  * Copyright 2023-2026 Joaquin M Lopez Munoz.
+ * Copyright 2026 Braden Ganetsky
  * Distributed under the Boost Software License, Version 1.0.
  * (See accompanying file LICENSE_1_0.txt or copy at
  * http://www.boost.org/LICENSE_1_0.txt)
@@ -186,10 +187,10 @@ namespace boost {
       {
       }
 
-
-      template <bool avoid_explicit_instantiation = true>
+      template <typename Key2,
+        typename std::enable_if<std::is_same<Key, Key2>::value, int>::type = 0>
       concurrent_flat_set(
-        unordered_flat_set<Key, Hash, Pred, Allocator>&& other)
+        unordered_flat_set<Key2, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))
       {
       }

include/boost/unordered/concurrent_node_map.hpp

@@ -2,6 +2,7 @@
  *
  * Copyright 2023 Christian Mazakas.
  * Copyright 2023-2026 Joaquin M Lopez Munoz.
+ * Copyright 2026 Braden Ganetsky
  * Distributed under the Boost Software License, Version 1.0.
  * (See accompanying file LICENSE_1_0.txt or copy at
  * http://www.boost.org/LICENSE_1_0.txt)
@@ -197,9 +198,10 @@ namespace boost {
       {
       }
 
-      template <bool avoid_explicit_instantiation = true>
+      template <typename Key2,
+        typename std::enable_if<std::is_same<Key, Key2>::value, int>::type = 0>
       concurrent_node_map(
-        unordered_node_map<Key, T, Hash, Pred, Allocator>&& other)
+        unordered_node_map<Key2, T, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))
       {
       }

include/boost/unordered/concurrent_node_set.hpp

@@ -2,6 +2,7 @@
  *
  * Copyright 2023 Christian Mazakas.
  * Copyright 2023-2026 Joaquin M Lopez Munoz.
+ * Copyright 2026 Braden Ganetsky
  * Distributed under the Boost Software License, Version 1.0.
  * (See accompanying file LICENSE_1_0.txt or copy at
  * http://www.boost.org/LICENSE_1_0.txt)
@@ -194,9 +195,10 @@ namespace boost {
       {
       }
 
-      template <bool avoid_explicit_instantiation = true>
+      template <typename Key2,
+        typename std::enable_if<std::is_same<Key, Key2>::value, int>::type = 0>
       concurrent_node_set(
-        unordered_node_set<Key, Hash, Pred, Allocator>&& other)
+        unordered_node_set<Key2, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))
       {
       }

test/CMakeLists.txt

@@ -170,5 +170,6 @@ foa_tests(TYPE compile NAME explicit_instantiation_tests SOURCES unordered/expli
 cfoa_tests(TYPE compile NAME explicit_instantiation_tests SOURCES cfoa/explicit_instantiation_tests.cpp)
 
 foa_tests(TYPE compile NAME conversion_operator_tests SOURCES unordered/conversion_operator_tests.cpp)
+cfoa_tests(TYPE compile NAME conversion_operator_tests SOURCES cfoa/conversion_operator_tests.cpp)
 
 endif()

test/Jamfile.v2

@@ -167,6 +167,7 @@ compile unordered/explicit_instantiation_tests.cpp : <define>BOOST_UNORDERED_FOA
 compile cfoa/explicit_instantiation_tests.cpp      :                                   : cfoa_explicit_instantiation_tests ;
 
 compile unordered/conversion_operator_tests.cpp : <define>BOOST_UNORDERED_FOA_TESTS : foa_conversion_operator_tests ;
+compile cfoa/conversion_operator_tests.cpp      :                                   : cfoa_conversion_operator_tests ;
 
 local FCA_EXCEPTION_TESTS =
   constructor_exception_tests

test/cfoa/conversion_operator_tests.cpp

@@ -0,0 +1,42 @@
+// Copyright 2026 Braden Ganetsky
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/static_assert.hpp>
+#include <boost/unordered/concurrent_flat_map.hpp>
+#include <boost/unordered/concurrent_flat_set.hpp>
+#include <boost/unordered/concurrent_node_map.hpp>
+#include <boost/unordered/concurrent_node_set.hpp>
+// Don't include the FOA headers here!
+
+using c_flat_map = boost::unordered::concurrent_flat_map<int, int>;
+using c_flat_set = boost::unordered::concurrent_flat_set<int>;
+using c_node_map = boost::unordered::concurrent_node_map<int, int>;
+using c_node_set = boost::unordered::concurrent_node_set<int>;
+
+struct constrained_template_converter
+{
+  struct dummy
+  {
+  };
+  template <class T, typename std::enable_if<
+                       std::is_constructible<T, dummy>::value, int>::type = 0>
+  operator T() const
+  {
+    return T{};
+  }
+};
+
+// Check whether the corresponding FOA container gets instantiated.
+// The FOA headers aren't included, so this would fail to compile if the FOA
+// container was instantiated.
+BOOST_STATIC_ASSERT(
+  (!std::is_constructible<c_flat_map, constrained_template_converter>::value));
+BOOST_STATIC_ASSERT(
+  (!std::is_constructible<c_flat_set, constrained_template_converter>::value));
+BOOST_STATIC_ASSERT(
+  (!std::is_constructible<c_node_map, constrained_template_converter>::value));
+BOOST_STATIC_ASSERT(
+  (!std::is_constructible<c_node_set, constrained_template_converter>::value));
+
+int main() { return 0; }