At the company I work at we created a class called 'RestrictedMap'. This provides the same interface as a regular std::map but will not allow you to use the [] operator. Some other functions have been provided to work with the class comfortably. Internally the class wraps an std::map.
I'm now trying to create a similar class that does the same for a boost::ptr_map, called 'RestrictedPointerMap'. For this I've created RestrictedMapBase that accepts, as a template argument, the type of map it should wrap and contains most of the implementation. Two classes derive of it and specify the type of map to be wrapped:
Here's the code, I haven't simplified the class for completeness but I will name the relevant functions later on.
RestrictedMap.h
#pragma once
#include <boost/ptr_container/ptr_map.hpp>
#include <boost/static_assert.hpp>
#include <boost/assign/list_of.hpp>
#include <boost/foreach.hpp>
#include <map>
/**
* Class that has the benefits of a map, but does not add an entry if it does not exists.
* Do not use RestrictedMapBase directly but use one of the derived classes (RestrictedMap or RestrictedPointerMap).
*/
template <typename MAP>
class RestrictedMapBase
{
public:
RestrictedMapBase(const MAP& map):
m_map(map)
{}
template<class InputIterator>
RestrictedMapBase(InputIterator first, InputIterator last):
m_map(first, last)
{}
RestrictedMapBase()
{}
/************************************************************************/
/* std::map interface */
/************************************************************************/
typedef typename MAP::iterator iterator;
typedef typename MAP::const_iterator const_iterator;
typedef typename MAP::value_type value_type;
typedef typename MAP::key_type key_type;
typedef typename MAP::mapped_type mapped_type;
typedef typename MAP::size_type size_type;
iterator begin() { return m_map.begin(); }
iterator end() { return m_map.end(); }
const_iterator begin() const { return m_map.begin(); }
const_iterator end() const { return m_map.end(); }
bool empty() const { return m_map.empty(); }
size_type size() const { return m_map.size(); }
iterator find(const key_type& key) { return m_map.find(key); }
const_iterator find(const key_type& key) const { return m_map.find(key); }
void clear() { m_map.clear(); }
void erase(iterator where) { m_map.erase(where); }
bool operator==(const typename RestrictedMapBase<MAP>& other) const { return m_map == other.m_map; }
bool operator!=(const typename RestrictedMapBase<MAP>& other) const { return m_map != other.m_map; }
bool operator<(const typename RestrictedMapBase<MAP>& other) const { return m_map < other.m_map; }
/************************************************************************/
/* extra */
/************************************************************************/
void erase(const key_type& key)
{
iterator iter(find(key));
assert(found(iter));
erase(iter);
}
void eraseIfExists(const key_type& key)
{
m_map.erase(key);
}
bool exists(const key_type& key) const
{
return found(find(key));
}
mapped_type& getValue(const key_type& key)
{
return const_cast<mapped_type&>(static_cast<const RestrictedMapBase<MAP>&> (*this).getValue(key));
}
const mapped_type& getValue(const key_type& key) const
{
const_iterator iter(find(key));
assert(found(iter));
return getData(iter);
}
mapped_type getValueIfExists(const key_type& key) const
{
BOOST_STATIC_ASSERT(boost::is_pointer<mapped_type>::value);
const_iterator iter(find(key));
if (found(iter)) {
return getData(iter);
} else {
return 0;
}
}
void setValue(const key_type& key, const mapped_type& value)
{
iterator iter(find(key));
assert(found(iter));
setData(iter, value);
}
void add(const key_type& key, const mapped_type& value)
{
assert(!exists(key));
insert(key, value);
}
void add(const RestrictedMapBase<MAP>& mapToAdd)
{
BOOST_FOREACH(value_type element, mapToAdd.m_map)
{
add(element.first, element.second);
}
}
void addOrReplace(const key_type& key, const mapped_type& value)
{
iterator iter(find(key));
if (found(iter)) {
setData(iter, value);
} else {
insert(key, value);
}
}
mapped_type* addDefaultConstructed(const key_type& key)
{
assert(!exists(key));
return &m_map[key];
}
private:
bool found(const const_iterator& iter) const
{
return iter != end();
}
const mapped_type& getData(const const_iterator& iter) const
{
return const_cast<const mapped_type&>(iter->second);
}
mapped_type& getData(const iterator& iter)
{
return const_cast<mapped_type&>(static_cast<const RestrictedMapBase<MAP>&>(*this).getData(iter));
}
void setData(const iterator& iter, const mapped_type& value)
{
getData(iter) = value;
}
virtual void insert(const key_type& key, const mapped_type& value) = 0;
protected:
MAP& getMap()
{
return m_map;
}
private:
MAP m_map;
};
template <typename KEYTYPE, typename DATATYPE>
class RestrictedMap: public RestrictedMapBase<std::map<KEYTYPE, DATATYPE> >
{
public:
RestrictedMap(const std::map<typename KEYTYPE, typename DATATYPE>& map): RestrictedMapBase(map)
{}
template<class InputIterator>
RestrictedMap(InputIterator first, InputIterator last): RestrictedMapBase(first, last)
{}
RestrictedMap()
{}
virtual void insert(const KEYTYPE& key, const DATATYPE& value)
{
getMap().insert(std::make_pair(key, value));
}
};
template <typename KEYTYPE, typename DATATYPE>
class RestrictedPointerMap: public RestrictedMapBase<boost::ptr_map<KEYTYPE, DATATYPE> >
{
public:
RestrictedPointerMap(const boost::ptr_map<typename KEYTYPE, typename DATATYPE>& map): RestrictedMapBase(map)
{}
template<class InputIterator>
RestrictedPointerMap(InputIterator first, InputIterator last): RestrictedMapBase(first, last)
{}
RestrictedPointerMap()
{}
virtual void insert(const KEYTYPE& key, DATATYPE* const& value)
{
/* boost::ptr_map::mapped_type does not equal the DATATYPE template parameter passed to it. Therefore this
* functions signature *looks* different from the RestrictedMapBase::insert signature */
getMap().insert(key, std::auto_ptr<DATATYPE>(value));
}
};
This works mostly, except when I want to call getValue on a RestrictedPointerMap. The function getData returns the correct value but after that it goes wrong in the getValue function. It returns a wrong pointer (as in the pointer is wrong).
Here is some code that reproduces the issue:
TestClass.h
#pragma once
class SomeClass
{
public:
SomeClass();
virtual ~SomeClass();
};
TestClass.cpp
#include "stdafx.h"
#include "TestClass.h"
#include <iostream>
SomeClass::SomeClass()
{
std::cout << "TestClass[" << this << "] created." << std::endl;
}
SomeClass::~SomeClass()
{
std::cout << "TestClass[" << this << "] deleted." << std::endl;
}
TestRestrictedPtrMap.cpp (main)
#include "stdafx.h"
#include "RestrictedMap.h"
#include "TestClass.h"
#include <boost/foreach.hpp>
int _tmain(int argc, _TCHAR* argv[])
{
typedef RestrictedPointerMap<int, SomeClass> MapType;
MapType theMap;
theMap.add(1, new SomeClass());
theMap.add(2, new SomeClass());
BOOST_FOREACH(MapType::value_type mapEntry, theMap) {
std::cout << mapEntry.first << " = " << mapEntry.second << std::endl;
}
SomeClass* oneClass = theMap.getValue(1);
std::cout << oneClass << std::endl;
SomeClass* twoClass = theMap.getValue(2);
std::cout << twoClass << std::endl;
std::cin.get();
return 0;
}
The output of this is:
TestClass[0078A318] created.
TestClass[0078A448] created.
1 = 0078A318
2 = 0078A448
0018FBD4
0018FBD4
TestClass[0078A318] deleted.
TestClass[0078A448] deleted.
I have no clue why it goes wrong. As far as I know the return value goes bad by magic.
Thanks in advance for any help,
Tom
You've got a dangling reference.
When you dereference a boost::ptr_map<Key, T>::iterator it constructs on-the-fly a boost::ptr_container_detail::ref_pair<Key, T *> initialised from the actual underlying iterator (a std::map<Key, void *>::iterator). This means that the T *& (or const T *&) returned from getData is referencing a member of a local temporary (the second member of iter->second):
const mapped_type& getData(const const_iterator& iter) const
{
return const_cast<const mapped_type&>(iter->second); // reference to a temporary
}
^^^^^^ *iter is a temporary value
This differs from a normal std::map, where *iter gives a reference to the value subobject of the node in the map's binary tree.
There's no easy solution without significantly changing your interface, as there is no actual T * object anywhere in memory to take a reference to. You might do better to change the signature of your RestrictedPointerMap to return the T mapped values by value-pointer or even by direct reference:
T *getValue(const key_type& key); // not T *&
const T *getValue(const key_type& key) const; // not const T *const &
// or
T &getValue(const key_type& key); // not T *&
const T &getValue(const key_type& key) const; // not const T *const &