How do you determine the size of the nodes created by a 'std::map' for use with 'boost::pool_allocator' (in a cross-platform way)?

Question

UPDATE

Per comments, answer, and additional research, I have come to the conclusion that there is typically no difference between a set

Answer 1

The problem is that you don't know the internal type which set uses for nodes.

While I haven't figured out how to determine this at compile time, you can write a tracing allocator which prints out the sizeof the node type when allocate is called, as in:

template
struct SimpleAllocator : private std::allocator
{
    using value_type = T;
    using pointer = typename std::allocator::pointer;
    using size_type = typename std::allocator::size_type;

    pointer allocate(size_type n)
    {   
        std::cout << "Allocator sizeof(T)==" << sizeof(T) << '\n';
        return std::allocator::allocate(n);
    }   

    void deallocate(pointer p, size_type n)
    { return std::allocator::deallocate(p, n); }
};

And a little test program (I'm testing with sets of ints):

std::set, SimpleAllocator> s;
s.insert(2);

On my system, I get output of:

Allocator sizeof(T)==32