MAINT-5232: Add LLHeteroMap to contain objects of unrelated classes.

1 files changed, 95 insertions, 0 deletions

diff --git a/indra/llcommon/llheteromap.h b/indra/llcommon/llheteromap.h
new file mode 100644
index 0000000000..9d6f303d08
--- /dev/null
+++ b/indra/llcommon/llheteromap.h
@@ -0,0 +1,95 @@
+/**
+ * @file   llheteromap.h
+ * @author Nat Goodspeed
+ * @date   2016-10-12
+ * @brief  Map capable of storing objects of diverse types, looked up by type.
+ * 
+ * $LicenseInfo:firstyear=2016&license=viewerlgpl$
+ * Copyright (c) 2016, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_LLHETEROMAP_H)
+#define LL_LLHETEROMAP_H
+
+#include <typeinfo>
+#include <utility>                  // std::pair
+#include <map>
+
+/**
+ * LLHeteroMap addresses an odd requirement. Usually when you want to put
+ * objects of different classes into a runtime collection of any kind, you
+ * derive them all from a common base class and store pointers to that common
+ * base class.
+ *
+ * LLInitParam::BaseBlock uses disturbing raw-pointer arithmetic to find data
+ * members in its subclasses. It seems that no BaseBlock subclass can be
+ * stored in a polymorphic class of any kind: the presence of a vtbl pointer
+ * in the layout silently throws off the reinterpret_cast arithmetic. Bad
+ * Things result. (Many thanks to Nicky D for this analysis!)
+ *
+ * LLHeteroMap collects objects WITHOUT a common base class, retrieves them by
+ * object type and destroys them when the LLHeteroMap is destroyed.
+ */
+class LLHeteroMap
+{
+public:
+    ~LLHeteroMap();
+
+    /// find or create
+    template <class T>
+    T& obtain()
+    {
+        // Look up map entry by typeid(T). We don't simply use mMap[typeid(T)]
+        // because that requires default-constructing T on every lookup. For
+        // some kinds of T, that could be expensive.
+        TypeMap::iterator found = mMap.find(&typeid(T));
+        if (found == mMap.end())
+        {
+            // Didn't find typeid(T). Create an entry. Because we're storing
+            // only a void* in the map, discarding type information, make sure
+            // we capture that type information in our deleter.
+            void* ptr = new T();
+            void (*dlfn)(void*) = &deleter<T>;
+            std::pair<TypeMap::iterator, bool> inserted =
+                mMap.insert(TypeMap::value_type(&typeid(T),
+                    TypeMap::mapped_type(ptr, dlfn)));
+            // Okay, now that we have an entry, claim we found it.
+            found = inserted.first;
+        }
+        // found->second is the std::pair; second.first is the void*
+        // pointer to the object in question. Cast it to correct type and
+        // dereference it.
+        return *(static_cast<T*>(found->second.first));
+    }
+
+private:
+    template <class T>
+    static
+    void deleter(void* p)
+    {
+        delete static_cast<T*>(p);
+    }
+
+    // Comparing two std::type_info* values is tricky, because the standard
+    // does not guarantee that there will be only one type_info instance for a
+    // given type. In other words, &typeid(A) in one part of the program may
+    // not always equal &typeid(A) in some other part. Use special comparator.
+    struct type_info_ptr_comp
+    {
+        bool operator()(const std::type_info* lhs, const std::type_info* rhs)
+        {
+            return lhs->before(*rhs);
+        }
+    };
+
+    // What we actually store is a map from std::type_info (permitting lookup
+    // by object type) to a void* pointer to the object PLUS its deleter.
+    typedef std::map<
+        const std::type_info*, std::pair<void*, void (*)(void*)>,
+        type_info_ptr_comp>
+    TypeMap;
+    TypeMap mMap;
+};
+
+#endif /* ! defined(LL_LLHETEROMAP_H) */