This shows how you can use pointer arithmetic to store the size of the allocated memory block in the pointer itself and use it to store how much memory is allocated. This does not include new[] or delete[] as well as malloc, calloc, realloc, free re-definitions (therefore the STL may allocate and use untracked memory)

allocateTest.cpp

#include <new>
#include <stdint.h>
#include <stdlib.h>
#include <cstdio>

#if 1
// Define these as old ones
#define _new new
#define _delete delete
#undef new
#undef delete

size_t allocatedMemory;

void * operator new(size_t ptrSize) throw(std::bad_alloc)
{
        // Allocate a pointer as a size_t with enough space to store
        // the amount of allocated memory needed..
        size_t *ptr = (size_t*)malloc(ptrSize+sizeof(size_t));

        // Debug
        printf("Allocated pointer of size: %ld\n", ptrSize);

        if(!ptr)
                throw std::bad_alloc();
        else
        {
                // Store the memory allocated in the ptr itself
                *ptr = ptrSize;
                allocatedMemory += ptrSize;
                // Return the pointer + offset of the size to the real allocated memory
                // the user intended.
                ptr++;
                return (void*) ptr;
        }
}

void operator delete(void *allocatedPTR) throw()
{
        // Get the memory chunk size from the pointer
        size_t *ptr = (size_t*)allocatedPTR;
        ptr--;
        size_t ptrSize = (size_t) *ptr;
        allocatedMemory -= ptrSize;

        printf("Deallocating pointer of size %ld\n", ptrSize);
        // Make sure we correct the offset of the pointer before
        // deallocating it
        free(ptr);
}
#endif

struct reallyBig;
struct reallyBig
{
        char bigarr[(1 << 16)];
        struct reallyBig *somePTR;
        double doubleDecker;
        long long int veryLongInt;
        struct reallyBig *bigArr[(1 << 16)];
};


int main()
{
        reallyBig *ptr = new reallyBig;
        reallyBig *ptr2 = new reallyBig;
        char *chr = new char;
        *chr = 'H';

        ptr->veryLongInt = 10000000;
        printf("Int: %lld\n", ptr->veryLongInt);
        printf("Char: %s\n", chr);
        printf("Total memory allocated: %ld\n", allocatedMemory);

        delete chr;
        delete ptr;
        delete ptr2;
        printf("Total memory allocated: %ld\n", allocatedMemory);
        return 0;
}