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