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Can I declare a very large array in a class, C ++

I am trying to write a class to store millions of 3D coordinate data. First, I tried to use a 3D array to store coordinate data.

#ifndef DUMPDATA_H
#define DUMPDATA_H
#define ATOMNUMBER 2121160
#include <string>
using namespace std;
class DumpData
{
public:
    DumpData(string filename);
    double m_atomCoords[ATOMNUMBER][3];
};
#endif // DUMPDATA_H

Then I compiled the program, but I got segfaults when I run the program on ubuntu 14.04 (64 bit) system. So I changed the 3D array to a vector by declaring:

vector < vector <double> > m_atomCoords;

Then the program worked. I'm just wondering if there are limitations in declaring very large arrays in a class?

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2 answers


In general, the stack is limited in size.

This will lead to a stack overflow:

int main() {
    DumpData x;
}


Until it is:

int main() {
    static DumpData x;
    std::unique_ptr<DumpData> y = std::make_unique<DumpData>();
}
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The stack is a very valuable and scarce resource, so I would use the heap to allocate big data.

If you have an array of 3D coordinates, instead of using it vector<vector<double>>

, I would simply define a class to represent the 3D point using only three separate data items, double

or a raw array of three double

s, like this:

class Point3D {
 private:
  double m_vec[3]; // X, Y and Z

  // or:
  // double x;
  // double y;
  // double z; 

 public:
  double X() const {
    return m_vec[0];
    // or:
    // return x;
  }   
  ... other setters/getters, etc.
};


and then i just use it as data item inside your class . std::vector<Point3D>

DumpData

(Class A Point3D

defined above has less overhead than a std::vector<double>

and also offers a higher level of semantics, so this is a better choice.)

With a default allocator, std::vector

allocates memory for a huge amount Point3D

from the heap (not from the stack), which works well and is also hidden from the client DumpData

, creating a nice simple public interface for the class DumpData

.

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