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/**
* @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) */
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