Allocating memory for a vector of vectors

Question

I'm trying to reserve space for a vector of vectors, but it doesn't work and throws the following error:

terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc

Every time I use a large enough number. A minimal version of what I have is below:

#include <vector>
#include <iostream>
using namespace std;


int main(){

  int base;
  cout << "Enter Base: ";
  cin >> base;

  int dimension;
  cout << "Enter Dimension: ";
  cin >> dimension;

  int perms = 1;
  for(int i=0; i<dimension; i++){
    perms *= base;
  } // This gets the number of permutations with repetition

  int length;
  cout << "Enter Length: ";
  cin >> length;

  float structSize = 1.0;

  for(float i=0.0; i<length; i++){
    structSize *= perms-i;
    structSize /= (i+1.0);
  } // This gets the number of combinations

  vector< vector< vector<double> > > allStructs;
  allStructs.reserve(structSize); 

  return 0;
}

It should work for large structSizes, but fails at base=3, dimension=4, length=6 which makes structSize=324,540,216. Is it possible for this to work?

Answer 1

You need to think about your memory use.

It should work for large structSizes, but fails at base=3, dimension=4, length=6 which makes structSize=324,540,216. Is it possible for this to work?

So what you're doing, at an abstract level, is allocating a data structure that contains 324,540,216 instances of a vector<vector<double>> object.

Here's what we know about vector objects:

• Its size must be at least 16 bytes; it needs to store a pointer, which in a 64-bit architecture, is probably 8 bytes, and it needs to store a size, which will probably also be 8 bytes.
• Its size is probably going to be significantly bigger, since the moment you instantiate the last vector<double> object, it's going to consume another [at-least-] 16 bytes each time you create one.

So just on the face of it, your allStructs.reserve(structSize) call is allocating 5 Gigabytes. It's probably allocating more than that, since the size of a vector's metadata could very well be larger than 16 bytes.

Answer 2

Declare your StructSize as double StructSize = 1.0; then it should " logically " work.

However, reserve() might not work due to possible memory limitations on your PC.