Boost bimap takes much memory

Question

I looked into this, but not helpful to me.

I have a boost bimap where I have 1 million entries of long long int. Theoretically it takes 1000000*8*2 bytes = 16 MB of memory. But I found that total memory used is 73 MB. I don't know what is going wrong.

Following is the example code:

main.cpp

#include <iostream>
#include <map>
#include "index.h"    

template<typename T>
struct MemoryMapAllocator : std::allocator<T> {
    typedef typename std::allocator<T>::pointer pointer;
    typedef typename std::allocator<T>::size_type size_type;
    template<typename U> struct rebind { typedef MemoryMapAllocator<U> other; };

    MemoryMapAllocator() {}

    template<typename U>
    MemoryMapAllocator(const MemoryMapAllocator<U>& u) : std::allocator<T>(u) {}

    pointer allocate(size_type size, std::allocator<void>::const_pointer = 0) {
        void* p = std::malloc(size * sizeof(T));
        if(p == 0)
            throw std::bad_alloc();
        return static_cast<pointer>(p);
    }
    void deallocate(pointer p, size_type) {
        std::free(p);
    }
};

typedef std::map<void*, std::size_t, std::less<void*>,
            MemoryMapAllocator<std::pair<void* const, std::size_t>>> MemoryMap;

MemoryMap& getMemoryMap() {
    static MemoryMap memMap;
    return memMap;
}

std::size_t totalAllocatedMemory() {
    std::size_t sum = 0;
    for(auto& e : getMemoryMap())
        sum += e.second;
    return sum;
}

void* operator new(std::size_t size) {
    void* mem = std::malloc(size == 0 ? 1 : size);

    if(mem == 0)
        throw std::bad_alloc();

    getMemoryMap()[mem] = size;
    return mem;
}

void operator delete(void* mem) noexcept {
    getMemoryMap().erase(mem);
    std::free(mem);
}


int main(){
    reference_index_hash();
     std::cout << "Memory allocated: " << totalAllocatedMemory() << " bytes." << std::endl;
     return 0;
}

index.cpp

#include <iostream>
#include <boost/unordered_map.hpp>
#include <boost/bimap.hpp>
#include <boost/bimap/unordered_set_of.hpp>
#include <boost/bimap/unordered_multiset_of.hpp>
#include <boost/bimap/set_of.hpp>
#include <boost/bimap/multiset_of.hpp>


namespace bimaps = boost::bimaps;
typedef boost::bimap<bimaps::unordered_set_of<unsigned long long int>,
        bimaps::unordered_multiset_of<unsigned long long int> > bimap_reference;
typedef bimap_reference::value_type position;
bimap_reference numbers;


/*
 * Creating a bimap structure.
 */

int reference_index_hash(){
    for (unsigned int i=0; i < 1000000; ++i ){
        numbers.insert(position(i, i+10));
    }
    std::cout << "bimap creation....done" << std::endl;
    return 0;
}

index.h

#ifndef INDEX_H_
#define INDEX_H_


#include<iostream>
#include <boost/unordered_map.hpp>
#include <boost/bimap.hpp>
#include <boost/bimap/unordered_set_of.hpp>
#include <boost/bimap/unordered_multiset_of.hpp>
#include <boost/bimap/set_of.hpp>
#include <boost/bimap/multiset_of.hpp>

extern int reference_index_hash();

namespace bimaps = boost::bimaps;

typedef boost::bimap<bimaps::unordered_set_of<unsigned long long int>,
        bimaps::unordered_multiset_of<unsigned long long int > > bimap_reference;
typedef bimap_reference::value_type position;
extern bimap_reference reference_index_vector;



#endif /* INDEX_H_ */

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