Implicit static variable in C++?

Question

I have an object SeatSelection. In this object, I declare a array of integer arrays as follows

int *rows[25];

My SeatSelection constructor initializes the rows variable as follows:

SeatSelection::SeatSelection(int start, int range){
  this->range = range;
  this->start = start;

  for(int i = 0; i < range; i++){
    rows[i] = new int[10];

  for(int j = 0; j < 10; j++)
    rows[i][j] = (j+1);
  }
}

In theory, I should now have an array of integer arrays of size 10 right?

In my driver class,

I am declaring two SeatSelection objects, both with different paramaters:

SeatSelection premium(1,5);
SeatSelection regular(6, 15);

premium should have 5 rows of 10 seats each = 50 seats.

regular should have 15 rows of 10 seats each = 150 seats.

Through some debugging, I found out that both these distinct SeatSelection objects are sharing the same rows pointer/array. I did not declare this variable as static. Why is this happening?

This is for an assignment where we are not allowed to use std::vector. Our arrays must be done "by hand." — Omar Darwish
– Omar Darwish, Commented Oct 18, 2013 at 21:45
@OmarDarwish: No. I want you to build an SSCCE that demonstrates the same problem. 20 lines of code ought to do it. — John Dibling
– John Dibling, Commented Oct 18, 2013 at 21:50
Here's something that will blow your mind. Since rows was a global, it actually did have implicit static lifetime! — John Dibling
– John Dibling, Commented Oct 18, 2013 at 21:58

Ben Voigt · Accepted Answer · 2013-10-18 21:52:26Z

1

To get a separate rows variable for each class instance, it's not sufficient to make the variable "not static".

You have to make it a class member to give it per-instance storage.

Other non-static variables have the storage duration determined by their scope. A non-static namespace-member (including the global namespace) variable has one copy for the entire program. A non-static local variable has one copy per invocation of the function.

answered Oct 18, 2013 at 21:52

Ben Voigt

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1 Comment

Ben Voigt Over a year ago

@John: Agreed. Your crystal ball worked quite well for this one.

Khulja Sim Sim · Accepted Answer · 2013-10-18 21:53:17Z

0

Looks like int *row[25] are not part of SeatSelection class. You may have row[] array locally / globally declared and each of SeatSelection object is using same space. After premium object writes to row[] memory, regular object overwrites it.

answered Oct 18, 2013 at 21:53

Khulja Sim Sim

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