size of double and float objects in a list are equal?

Question

I am wondering if the size of float and double objects are equal from std::list point of view?

I've allocated 5-million Real(alias float or double) objects in a std::list and used Valgrind to monitor memory usage.

in both cases the used memory is equal although the size of a 'double' (8 bytes) is double the size if a 'float' object (4 bytes)!

Btw, when I allocate memory for the same amount of objects using 'new' operator, the memory usage of the double array is double the usage of the float array, which seems about right. I was expecting the same using std::list too.

I am using gcc 4.6.2, on Fedora 16.x86_64.

Any idea to help me figure the mystery is appreciated.

here is the code I wrote for test

#include <iostream>
#include <list>

typedef double Real;

int main(int argc, char** argv)
{
    std::list<Real> pts;
    int k;

    int npts = 5000000; // 5 mil

    std::cout << "sizeof(Real): " << sizeof(Real) << std::endl;
    for(k=0; k < npts;++k)
        pts.push_back(1.0);

    return 0;

}

if I define Real <- double the Valgrind output is

==15335== Memcheck, a memory error detector
==15335== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==15335== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright info
==15335== Command: /home/soheil/Workspace/tbin/test_memory_usage
==15335== 
sizeof(Real): 8
==15335== 
==15335== HEAP SUMMARY:
==15335==     in use at exit: 616 bytes in 6 blocks
==15335==   total heap usage: 5,000,053 allocs, 5,000,047 frees, 120,015,245 bytes allocated
==15335== 
==15335== LEAK SUMMARY:
==15335==    definitely lost: 0 bytes in 0 blocks
==15335==    indirectly lost: 0 bytes in 0 blocks
==15335==      possibly lost: 0 bytes in 0 blocks
==15335==    still reachable: 616 bytes in 6 blocks
==15335==         suppressed: 0 bytes in 0 blocks
==15335== Rerun with --leak-check=full to see details of leaked memory
==15335== 
==15335== For counts of detected and suppressed errors, rerun with: -v
==15335== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)

if I define Real <- float the Valgrind output is

==15252== Memcheck, a memory error detector
==15252== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==15252== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright info
==15252== Command: /home/soheil/Workspace/tbin/test_memory_usage
==15252== 
sizeof(Real): 4
==15252== 
==15252== HEAP SUMMARY:
==15252==     in use at exit: 616 bytes in 6 blocks
==15252==   total heap usage: 5,000,053 allocs, 5,000,047 frees, 120,015,245 bytes allocated
==15252== 
==15252== LEAK SUMMARY:
==15252==    definitely lost: 0 bytes in 0 blocks
==15252==    indirectly lost: 0 bytes in 0 blocks
==15252==      possibly lost: 0 bytes in 0 blocks
==15252==    still reachable: 616 bytes in 6 blocks
==15252==         suppressed: 0 bytes in 0 blocks
==15252== Rerun with --leak-check=full to see details of leaked memory
==15252== 
==15252== For counts of detected and suppressed errors, rerun with: -v
==15252== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)

Answer 1

Each element in a std::list<T> is a linked-list node, so it's a struct containing two pointers, as well as the payload data of type T. For instance, for GCC 4.1.2, it's as follows:

  struct _List_node_base
  {
    _List_node_base* _M_next;
    _List_node_base* _M_prev;

    // *** Non-virtual member functions ***
  };

  template<typename _Tp>
    struct _List_node : public _List_node_base
    {
      _Tp _M_data;
    };

The size allocated will be the size of that struct; if T is small enough then you may be seeing the figures dominated by struct padding.

So with the GCC definition, that's two 64-bit pointers (so 16 bytes), plus 4 or 8 bytes T, padded up to 8 bytes, so 24 bytes in total, which matches what you're measuring.

To test the theory, try changing Real to be float[2] or double[2].