memory-size of boost::multi_array

Question

I have a boost::multi_array of 3 dimensions

 boost::multi_array<Struct, 3>* newArr = new boost::multi_array<Struct, 3>(boost::extents[x][y][z], boost::fortran_storage_order())

Is there method to calculate the size of newArr or should I just use

sizeof(Struct)*x*y*z ?

Would they be the same? (I expect the multi_array to have little bit of controller-data)

Answer 1

You should be able to use

sizeof(Struct)*x*y*z + sizeof(boost::multi_array<Struct, 3>)

Let me search a bit whether the documentation reveals an easier/more generic approach.

Also, I like to just not bother and use the output of e.g. valgrind --tool=massif to know exactly what is allocated where. This would then also give the relevant results if you had, e.g.

struct Struct {
    std::string x;
    char const* my_data;
};

Here's sample output from Massif when using a Struct with just a std::string member:

#include <boost/multi_array.hpp>
struct Struct {
    std::string x;
};

int main() {
    int x=300,y=400,z=400;
    boost::multi_array<Struct, 3>* newArr = new boost::multi_array<Struct, 3>(boost::extents[x][y][z], boost::fortran_storage_order());
}

This results in e.g. on my system 384,000,160 bytes, which is exactly what's printed when you add std::cout << 300*400*400*sizeof(Struct)+sizeof(*newArr);

/tmp$ valgrind --tool=massif --detailed-freq=1 ./test
/tmp$ ms_print massif.out.32149 
--------------------------------------------------------------------------------
Command:            ./test
Massif arguments:   --detailed-freq=1
ms_print arguments: massif.out.32149
--------------------------------------------------------------------------------


    MB
366.2^                                                                       @
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   0 +----------------------------------------------------------------------->Mi
     0                                                                   1.273

Number of snapshots: 3
 Detailed snapshots: [0, 1, 2]

--------------------------------------------------------------------------------
  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
--------------------------------------------------------------------------------
  0              0                0                0             0            0
00.00% (0B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc.

--------------------------------------------------------------------------------
  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
--------------------------------------------------------------------------------
  1      1,332,810              168              160             8            0
95.24% (160B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc.
->95.24% (160B) 0x400ADA: main (test.cpp:8)

--------------------------------------------------------------------------------
  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
--------------------------------------------------------------------------------
  2      1,334,836      384,004,208      384,000,160         4,048            0
100.00% (384,000,160B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc.
->100.00% (384,000,000B) 0x401722: __gnu_cxx::new_allocator<Struct>::allocate(unsigned long, void const*) (new_allocator.h:94)
| ->100.00% (384,000,000B) 0x40138D: boost::multi_array<Struct, 3ul, std::allocator<Struct> >::allocate_space() (multi_array.hpp:474)
|   ->100.00% (384,000,000B) 0x400E52: boost::multi_array<Struct, 3ul, std::allocator<Struct> >::multi_array(boost::detail::multi_array::extent_gen<3ul> const&, boost::general_storage_order<3ul> const&) (multi_array.hpp:195)
|     ->100.00% (384,000,000B) 0x400AEB: main (test.cpp:8)
|       
->00.00% (160B) in 1+ places, all below ms_print's threshold (01.00%)