Question

When I type in the foll. code, I get the output as 1073741823.

#include <iostream>
#include <vector>
using namespace std;
int main()
{
  vector <int> v;
  cout<<v.max_size();
  return 0;
}

However when I try to resize the vector to 1,000,000,000, by v.resize(1000000000); the program stops executing. How can I enable the program to allocate the required memory, when it seems that it should be able to?

I am using MinGW in Windows 7. I have 2 GB RAM. Should it not be possible?
In case it is not possible, can’t I declare it as an array of integers and get away? BUt even that doesn’t work.

Another thing is that, suppose I would use a file(which can easily handle so much data ).
How can I let it read and write and the same time.
Using fstream file("file.txt', ios::out | ios::in ); doesn’t create a file, in the first place. But supposing the file exists, I am unable to use to do reading and writing simultaneously.
WHat I mean is this :
Let the contents of the file be 111111
Then if I run : –

#include <fstream>
#include <iostream>
using namespace std;
int main()
{
  fstream file("file.txt",ios:in|ios::out);
  char x;
  while( file>>x)
  {
    file<<'0';
  }
 return 0;
}

Shouldn’t the file’s contents now be 101010 ? Read one character and then overwrite the next one with 0 ? Or incase the entire contents were read at once into some buffer, should there not be atleast one 0 in the file ? 1111110 ?
But the contents remain unaltered. Please explain.
Thank you.

Answer 1

1. A 32-bit process can only address 4GB address space at a single time. Usually, plenty of this 4GB address space is used to map other stuff. Your vector is going to take too much contiguous address space (4 billion bytes) which is not likely to be available.

2. You should memory map the file. See mmap.