Custom allocator fails for std::map - c++

Question

I'm trying to use custom allocator with C++ STL containers, and it works with vector, but fails with map. Some strange error regarding mmap_allocator<std::_Rb_tree_node<std::pair<const int, int> > > and not using mmap_allocator<std::pair<const int, int> > as I was expecting
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/map:60,
from 4.cpp:2:
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_tree.h: In member function ‘_Alloc std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::get_allocator() const [with _Key = int, _Val = std::pair<const int, int>, _KeyOfValue = std::_Select1st<std::pair<const int, int> >, _Compare = std::less<int>, _Alloc = mmap_allocator<std::pair<const int, int> >]’:
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_tree.h:383: instantiated from ‘void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_destroy_node(std::_Rb_tree_node<_Val>*) [with _Key = int, _Val = std::pair<const int, int>, _KeyOfValue = std::_Select1st<std::pair<const int, int> >, _Compare = std::less<int>, _Alloc = mmap_allocator<std::pair<const int, int> >]’
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_tree.h:972: instantiated from ‘void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_erase(std::_Rb_tree_node<_Val>*) [with _Key = int, _Val = std::pair<const int, int>, _KeyOfValue = std::_Select1st<std::pair<const int, int> >, _Compare = std::less<int>, _Alloc = mmap_allocator<std::pair<const int, int> >]’
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_tree.h:614: instantiated from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::~_Rb_tree() [with _Key = int, _Val = std::pair<const int, int>, _KeyOfValue = std::_Select1st<std::pair<const int, int> >, _Compare = std::less<int>, _Alloc = mmap_allocator<std::pair<const int, int> >]’
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_map.h:87: instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_tree.h:354: error: no matching function for call to ‘mmap_allocator<std::pair<const int, int> >::mmap_allocator(const mmap_allocator<std::_Rb_tree_node<std::pair<const int, int> > >&)’
4.cpp:37: note: candidates are: mmap_allocator<T>::mmap_allocator(const mmap_allocator<T>&) [with T = std::pair<const int, int>]
4.cpp:36: note: mmap_allocator<T>::mmap_allocator() [with T = std::pair<const int, int>]
Here is the code:
#include <vector>
#include <map>
#include <stdio.h>
static size_t alloc;
template <typename T>
class mmap_allocator: public std::allocator<T>
{
public:
typedef size_t size_type;
typedef T* pointer;
typedef const T* const_pointer;
template<typename _Tp1>
struct rebind
{
typedef mmap_allocator<_Tp1> other;
};
pointer allocate(size_type n, const void *hint=0)
{
fprintf(stderr, "Alloc %d bytes.\n", n);
alloc += n;
return std::allocator<T>::allocate(n, hint);
}
void deallocate(pointer p, size_type n)
{
fprintf(stderr, "Dealloc %d bytes (%p).\n", n, p);
alloc -= n;
return std::allocator<T>::deallocate(p, n);
}
mmap_allocator() throw(): std::allocator<T>() { fprintf(stderr, "Hello allocator!\n"); }
mmap_allocator(const mmap_allocator &a) throw(): std::allocator<T>(a) { }
~mmap_allocator() throw() { }
};
int main(){
std::vector<int, mmap_allocator<int> > int_vec(1024, 0, mmap_allocator<int>());
std::map<int, int, std::less<int>, mmap_allocator<std::pair<int,int> > > x;
x[1] = 2;
printf("s=%lu\n", alloc);
return 0;
}
Linux, gcc 4.4.6.

Answer 1

I haven't tried fixing it but it seems you haven't defined a constructor which takes an allocator instantiation with a different template argument. That is, you are missing something like
template <typename T>
template <typename O>
mmap_allocator<T>::mmap_allocator(mmap_allocator<O> const& other) {
...
}
From the looks of it, the error stems from trying to construct an allocator type obtained from rebind with some other allocator.