CODEWITHSUNDEEP
c++arraysclassbigdata
Xuhang
Xuhang

Reputation: 495

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

I am trying to write a class to store millions 3D coordinates data. At the first, I tried to use a 3D array to store those coordinates 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 in ubuntu 14.04 system (64 bit). So I changed the 3D array to vector by declaring:

vector < vector <double> > m_atomCoords;

Then the program worked. I am just wondering are there limitations of declaring very large arrays in a class?

Upvotes: 4

Views: 1896

Answers (2)

Mr.C64
Mr.C64

Reputation: 42924

The stack is a very precious and scarce resource, so I'd just use the heap to allocate big data.

If you have an array of 3D coordinates, instead of using a vector<vector<double>>, I'd just define a class to represent a 3D point, using just three separate double data members, or a raw array of three doubles, e.g.:

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'd just use a std::vector<Point3D> as a data member inside your DumpData class.

(A Point3D class defined as above has less overhead than a std::vector<double>, and also offers an higher level of semantics, so it's a better choice.)

With the default allocator, std::vector will allocate the memory for the huge number of Point3Ds from the heap (not from the stack), which works well, and it is also hidden from the client of DumpData, making a nice simple public interface for the DumpData class.

Upvotes: 2

Bill Lynch
Bill Lynch

Reputation: 81916

In general, the stack has a limited size.

This will likely cause a stack overflow:

int main() {
    DumpData x;
}

While these won't:

int main() {
    static DumpData x;
    std::unique_ptr<DumpData> y = std::make_unique<DumpData>();
}

Upvotes: 6

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