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#include <unordered_map>
#include <memory>
 
template<typename T>
class TestAllocator;
 
template<>
class TestAllocator<void>
{
public:
  typedef void * pointer;
  typedef const void * const_pointer;
  typedef void value_type;
  typedef size_t size_type;
  typedef ptrdiff_t difference_type;
  //typedef T & reference; not possible for void
  //typedef const T & const_reference; not possible for void
 
  template<typename U>
  struct rebind
  {
    typedef TestAllocator<U> other;  
  };
};
 
template<typename T>
class TestAllocator
{
public:
  typedef T * pointer;
  typedef const T * const_pointer;
  typedef T & reference;
  typedef const T & const_reference;
  typedef T value_type;
  typedef size_t size_type;
  typedef ptrdiff_t difference_type;
 
  template<typename U>
  struct rebind
  {
    typedef TestAllocator<U> other;  
  };
 
  TestAllocator() { }
  template<typename U>
  TestAllocator(const TestAllocator<U> &other) { }
 
  pointer allocate(int n, pointer hint=0) { return new T[n]; }
  pointer deallocate(pointer p, size_t n) { delete[] p; }
  pointer address(reference x) { return &x; } // hack! should be std::addressof(x);
  void construct(pointer p, const_reference val) { new (p) value_type(val); }
  void destroy(pointer p) { p->~value_type(); }
};
 
struct Foo
{
    int a, b;
    bool operator <(const Foo &r) const
    {
        return a < r.a;
    }
};
 
struct Bar { float a, b; };
 
struct TrivialHash
{
    size_t operator()(const Foo &a) const
    {
        return a.a;
    }
};
 
struct TrivialEqual
{
    bool operator()(const Foo &a, const Foo &b) const
    {
        return a.a == b.a;
    }
};
 
typedef std::unordered_map<
  Foo, Bar,
  TrivialHash,
  TrivialEqual,
  TestAllocator<void>
> TestContainer;
 
int main()
{
    TestContainer x;
    x.insert(TestContainer::value_type(Foo(), Bar()));
    return 0;
}
 

In file included from /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/unordered_map:45:0,
                 from prog.cpp:1:
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/hashtable.h: In instantiation of 'std::_Hashtable<Foo, std::pair<const Foo, Bar>, TestAllocator<void>, std::_Select1st<std::pair<const Foo, Bar> >, TrivialEqual, TrivialHash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true>':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/unordered_map.h:49:5:   instantiated from 'std::__unordered_map<Foo, Bar, TrivialHash, TrivialEqual, TestAllocator<void>, false>'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/unordered_map.h:222:5:   instantiated from 'std::unordered_map<Foo, Bar, TrivialHash, TrivialEqual, TestAllocator<void> >'
prog.cpp:91:19:   instantiated from here
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/hashtable.h:140:59: error: no type named 'reference' in 'class TestAllocator<void>'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/hashtable.h:141:59: error: no type named 'const_reference' in 'class TestAllocator<void>'

Standard output is empty