Custom Memory Allocator for STL map

Question

This question is about construction of instances of custom allocator during insertion into a std::map.

Here is a custom allocator for std::map<int,int> along with a small program that uses it:

#include <stddef.h>
#include <stdio.h>
#include <map>
#include <typeinfo>

class MyPool {
public:
  void * GetNext() {
    return malloc(24);
  }
  void Free(void *ptr) {
    free(ptr);
  }
};

template<typename T>
class MyPoolAlloc {
public:
  static MyPool *pMyPool;

  typedef size_t     size_type;
  typedef ptrdiff_t  difference_type;
  typedef T*         pointer;
  typedef const T*   const_pointer;
  typedef T&         reference;
  typedef const T&   const_reference;
  typedef T          value_type;

  template<typename X>
  struct rebind
  { typedef MyPoolAlloc<X> other; };

  MyPoolAlloc() throw() {
    printf("-------Alloc--CONSTRUCTOR--------%08x %32s\n", this, typeid(T).name());
  }

  MyPoolAlloc(const MyPoolAlloc&) throw()  {
    printf(" Copy Constructor ---------------%08x %32s\n", this, typeid(T).name());
  }

  template<typename X>
  MyPoolAlloc(const MyPoolAlloc<X>&) throw() {
    printf(" Construct T Alloc from X Alloc--%08x %32s %32s\n", this, typeid(T).name(), typeid(X).name());
  }

  ~MyPoolAlloc() throw() {
    printf(" Destructor ---------------------%08x %32s\n", this, typeid(T).name());
  };

  pointer address(reference __x) const { return &__x; }

  const_pointer address(const_reference __x) const { return &__x; }

  pointer allocate(size_type __n, const void * hint = 0) {
    if (__n != 1)
      perror("MyPoolAlloc::allocate: __n is not 1.\n");
    if (NULL == pMyPool) {
      pMyPool = new MyPool();
      printf("======>Creating a new pool object.\n");
    }
    return reinterpret_cast<T*>(pMyPool->GetNext());
  }

  //__p is not permitted to be a null pointer
  void deallocate(pointer __p, size_type __n) {
    pMyPool->Free(reinterpret_cast<void *>(__p));
  }

  size_type max_size() const throw() {
    return size_t(-1) / sizeof(T);
  }

  void construct(pointer __p, const T& __val) {
    printf("+++++++ %08x %s.\n", __p, typeid(T).name());
    ::new(__p) T(__val);
  }

  void destroy(pointer __p) {
    printf("-+-+-+- %08x.\n", __p);
    __p->~T();
  }
};

template<typename T>
inline bool operator==(const MyPoolAlloc<T>&, const MyPoolAlloc<T>&) {
  return true;
}

template<typename T>
inline bool operator!=(const MyPoolAlloc<T>&, const MyPoolAlloc<T>&) {
  return false;
}

template<typename T>
MyPool* MyPoolAlloc<T>::pMyPool = NULL;

int main(int argc, char *argv[]) {

  std::map<int, int, std::less<int>, MyPoolAlloc<std::pair<const int,int> > > m;
  //random insertions in the map
  m.insert(std::pair<int,int>(1,2));
  m[5] = 7;
  m[8] = 11;
  printf("======>End of map insertions.\n");
  return 0;
}

Here is the output of this program:

-------Alloc--CONSTRUCTOR--------bffcdaa6                     St4pairIKiiE
 Construct T Alloc from X Alloc--bffcda77  St13_Rb_tree_nodeISt4pairIKiiEE                     St4pairIKiiE
 Copy Constructor ---------------bffcdad8  St13_Rb_tree_nodeISt4pairIKiiEE
 Destructor ---------------------bffcda77  St13_Rb_tree_nodeISt4pairIKiiEE
 Destructor ---------------------bffcdaa6                     St4pairIKiiE
======>Creating a new pool object.
 Construct T Alloc from X Alloc--bffcd9df                     St4pairIKiiE  St13_Rb_tree_nodeISt4pairIKiiEE
+++++++ 0985d028 St4pairIKiiE.
 Destructor ---------------------bffcd9df                     St4pairIKiiE
 Construct T Alloc from X Alloc--bffcd95f                     St4pairIKiiE  St13_Rb_tree_nodeISt4pairIKiiEE
+++++++ 0985d048 St4pairIKiiE.
 Destructor ---------------------bffcd95f                     St4pairIKiiE
 Construct T Alloc from X Alloc--bffcd95f                     St4pairIKiiE  St13_Rb_tree_nodeISt4pairIKiiEE
+++++++ 0985d068 St4pairIKiiE.
 Destructor ---------------------bffcd95f                     St4pairIKiiE
======>End of map insertions.
 Construct T Alloc from X Alloc--bffcda23                     St4pairIKiiE  St13_Rb_tree_nodeISt4pairIKiiEE
-+-+-+- 0985d068.
 Destructor ---------------------bffcda23                     St4pairIKiiE
 Construct T Alloc from X Alloc--bffcda43                     St4pairIKiiE  St13_Rb_tree_nodeISt4pairIKiiEE
-+-+-+- 0985d048.
 Destructor ---------------------bffcda43                     St4pairIKiiE
 Construct T Alloc from X Alloc--bffcda43                     St4pairIKiiE  St13_Rb_tree_nodeISt4pairIKiiEE
-+-+-+- 0985d028.
 Destructor ---------------------bffcda43                     St4pairIKiiE
 Destructor ---------------------bffcdad8  St13_Rb_tree_nodeISt4pairIKiiEE

Last two columns of the output show that an allocator for std::pair<const int, int> is constructed everytime there is a insertion into the map. Why is this necessary? Is there a way to suppress this?

Thanks!

Edit: This code tested on x86 machine with g++ version 4.1.2. If you wish to run it on a 64-bit machine, you'll have to change at least the line return malloc(24). Changing to return malloc(48) should work.

Answer 1

It is so because the allocator is for std::pair<const int, int> but the implementation actually needs to allocate a more complex data structure, of which that is a member. Whilst I'd expect the actual allocator needed to be constructed and cached, it is not illegal for it to be re-constructed every time. This is an implementation detail you cannot escape without changing your implementation. The actual allocator type that is created is St13_Rb_tree_nodeISt4pairIKiiEE (mangled name).