C++ Variables bigger than the stack (stack overflow)

Question

I need to create an array like this

double grid [15000][40];

but the stack in Visual Studio 2012 is only of 1MB. How can I use variables like this or bigger? This mean that if I create a

std::vector<int>

and I push_back 600 000 times it goes in stack overflow? This seems a big limitation, how can be solved? Thank you in advance.

The contents of a `std::vector` don't live on the stack. – Oliver Charlesworth Feb 23 '14 at 17:58 — Oliver Charlesworth, Feb 23 '14 at 17:58
I don't see how .NET or Java tags are relevant here. – Lightness Races in Orbit Feb 23 '14 at 17:58 — Lightness Races in Orbit, Feb 23 '14 at 17:58
Thanks for formatting; that's _much_ better! – Lightness Races in Orbit Feb 23 '14 at 17:59 — Lightness Races in Orbit, Feb 23 '14 at 17:59
Dynamic arrays dont go stack. – huseyin tugrul buyukisik Feb 23 '14 at 17:59 — huseyin tugrul buyukisik, Feb 23 '14 at 17:59

score 3 · Accepted Answer · answered Feb 23 '14 at 18:00

Large objects should have either static or dynamic storage duration.

Static:

int a[1000000];

void f()
{
    a[3] = 12;    // fine
}

Beware of shared, concurrent accesses to the static memory, though.

Dynamic (but managed properly by a suitable class):

void f()
{
    std::vector<int> a(1000000);   // dynamic objects managed by std::vector
    a[3] = 12;
}

Here each function call will create and manage its own dynamic allocation (and the complexities of concurrency are delegated to the memory allocator, so you don't have to think about those).

score 2 · Answer 2 · answered Feb 23 '14 at 17:58

2

There is no problem here.

This mean that if I create a std::vector and I push_back 600 000 times it goes in stack overflow? This seems a big limitation

No, because vectors elements do not have automatic storage duration (they don't "live on the stack"). They couldn't.

how can be solved

There is nothing to solve. Vector elements are dynamically allocated.

answered Feb 23 '14 at 17:58

Lightness Races in Orbit

378,754
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1,055

I'd like to use "double grid [15000][40]" not a vector, how can I do? – Nisba Feb 23 '14 at 18:00
2

@user3343783: If you want that on the stack: (a) no you don't (b) you shouldn't (c) you can't (d) don't. You can get around it with a `static` object, but I still wouldn't. – Lightness Races in Orbit Feb 23 '14 at 18:00
So use `vector> grid`; and `grid.resize(15000);`, then `for(i = 0; i < 15000; i++) grid[i].resize(4);`. – Mats Petersson Feb 23 '14 at 18:01
Or a 1D vector with faked 2D indexing over the top – Lightness Races in Orbit Feb 23 '14 at 18:04
Or any one of these suggestions here (you have choice between using vectors, using contiguous 2d array elements,...) : http://stackoverflow.com/questions/21943621/how-to-create-a-contiguous-2d-array-in-c/21944048#21944048 – PaulMcKenzie Feb 23 '14 at 18:06
@PaulMcKenzie: Or that, though it's not very RAII-like. – Lightness Races in Orbit Feb 23 '14 at 18:07
1

@LightnessRacesinOrbit You should have taken that all the way to _z_ ;) – Captain Obvlious Feb 23 '14 at 18:09
@LightnessRacesinOrbit - Yes, didn't have time to wrap in a class, but I leave that as an exercise... – PaulMcKenzie Feb 23 '14 at 18:09
thank you very much, it works perfectly. Just one last thing: why "std::array < std::array , M> grid;" crashes? does it operate in the stack too? – Nisba Feb 23 '14 at 18:11
1

@user3343783 - If you look at the implementation for std::array, you should see that one of the data members is a "regular" array. So yes, you get the same situation with std::array as you did with your original attempt. – PaulMcKenzie Feb 23 '14 at 19:00
@user3343783: Yes; `std::array` is a thin wrapper over normal arrays. – Lightness Races in Orbit Feb 23 '14 at 19:37

score 0 · Answer 3 · answered Feb 23 '14 at 18:01

0

You can define this array as

static double grid [15000][40];

As for std::vector then it allocates memory for its elements in the heap not in the stack.

answered Feb 23 '14 at 18:01

Vlad from Moscow

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score 0 · Answer 4 · answered Feb 23 '14 at 18:18

memory of vector is heap allocated, you don't have to worry about the stack. Instead of doing push_back, you can use the member function resize :

 std::vector<int> grid;
 grid.resize(15000*40);

or even better, you can use a unique pointer if the grid has a fixed size

score 0 · Answer 5 · answered Feb 23 '14 at 18:20

If you don't want to use std::vector for whatever reason, an alternative solution is:

int main()
{
  int (*grid)[40] = new int[15000][40];

  // work with grid
  grid[0][0] = 0.0;
  grid[14999][39] = 42;

  delete [] grid;
}

The usual caveats apply about raw pointers to dynamic storage causing memory leaks if the scope is terminated by an exception, so that the delete never executes.

C++ Variables bigger than the stack (stack overflow)

5 Answers5