Implement std::hash<boost::intrusive_ptr<T>> for old Boost versions

Boost only implements it iself starting from version 1.74, but a specialization
of std::hash<> can be added trivially to allow the use of
std::unordered_set<boost::intrusive_ptr<T>> and
std::unordered_map<boost::intrusive_ptr<K>, V>.

Being unable to use such types already came up a few types in the past, often
resulting in the use of raw pointer instead which always involves an additional
"is this safe?"/"could the object go out of scope?" discussion. This commit
simply solves this for the future by simply allowing the use of intrusive_ptr
in unordered containers.

lib/base/CMakeLists.txt

@@ -38,6 +38,7 @@ set(base_SOURCES
   filelogger.cpp filelogger.hpp filelogger-ti.hpp
   function.cpp function.hpp function-ti.hpp function-script.cpp functionwrapper.hpp
   initialize.cpp initialize.hpp
+  intrusive-ptr.hpp
   io-engine.cpp io-engine.hpp
   journaldlogger.cpp journaldlogger.hpp journaldlogger-ti.hpp
   json.cpp json.hpp json-script.cpp

lib/base/intrusive-ptr.hpp

@@ -0,0 +1,22 @@
+/* Icinga 2 | (c) 2025 Icinga GmbH | GPLv2+ */
+
+#pragma once
+
+#include "base/i2-base.hpp"
+#include <memory>
+#include <boost/smart_ptr/intrusive_ptr.hpp>
+#include <boost/version.hpp>
+
+// std::hash is only implemented starting from Boost 1.74. Implement it ourselves for older version to allow using
+// boost::intrusive_ptr inside std::unordered_set<> or as the key of std::unordered_map<>.
+// https://github.com/boostorg/smart_ptr/commit/5a18ffdc5609a0e64b63e47cb81c4f0847e0c087
+#if BOOST_VERSION < 107400
+template<class T>
+struct std::hash<boost::intrusive_ptr<T>>
+{
+	std::size_t operator()(const boost::intrusive_ptr<T>& ptr) const noexcept
+	{
+		return std::hash<T*>{}(ptr.get());
+	}
+};
+#endif /* BOOST_VERSION < 107400 */

lib/base/object.hpp

@@ -5,6 +5,7 @@
 
 #include "base/i2-base.hpp"
 #include "base/debug.hpp"
+#include "base/intrusive-ptr.hpp"
 #include <boost/smart_ptr/intrusive_ptr.hpp>
 #include <atomic>
 #include <cstddef>

lib/base/shared.hpp

@@ -4,6 +4,7 @@
 #define SHARED_H
 
 #include "base/atomic.hpp"
+#include "base/intrusive-ptr.hpp"
 #include <boost/smart_ptr/intrusive_ptr.hpp>
 #include <cstdint>
 #include <utility>