CODEWITHSUNDEEP
c++pointersmemory-managementvector
linuxfever
linuxfever

Reputation: 3823

Deleting a pointer of vectors

I am trying to write a very simple piece of code to create a 3x2 matrix using STL vectors.
This is what I have:

#include <vector>
using namespace std;

int main ()
{

  int i;

  vector<int> * x = new vector<int> [3];

  for (i = 0; i < 3; i++)
       x[i] = vector<int> (2); 

  delete x;

  return 0;

}

The problem is that every time I run the program, it crashes. If I remove the delete x then it works fine, but will probably result in memory leaks.

I know that this is probably not the best way to create a matrix and there is a battalion of choices out there, but I would just like to know why the above program crashes every time.

Also, replacing every occurrence of vector<int> with say int, then all work fine again.

Upvotes: 1

Views: 205

Answers (4)

juanchopanza
juanchopanza

Reputation: 227390

You need to use delete [], since you have a dynamically allocated array. The fact that you have vectors has nothing to do with it.

You could greatly simplify your code by using a vector<vector<int>>:

vector<vector<int>> v(3); // holds three empty vector<int>

Or even

vector<vector<int>> v(3, std::vector<int>(2)); // holds 3 size 2 vector<int>

Upvotes: 3

Kiril Kirov
Kiril Kirov

Reputation: 38153

You're using "array" new, so you need the same for delete:

delete[] x;
//....^^

Using delete x; here is undefined bahaviour, so anything could happen. You're lucky, that your program crashes :)

Don't do this, unless you have really good reason for this. You may have

std::vector< std::vector< int > > x;

Then your code would become:

std::vector< std::vector< int > > x( 3 );
for( unsigned int ii = 0; ii < x.size(); ++ii )
{
    x[ ii ].resize( 2 );
}

Or even:

std::vector< std::vector< int > > x( 3, std::vector< int >( 2 ) );

I'd do this like this:

typedef std::vector< int > SomeLogicalName; // or just IntVector
std::vector< SomeLogicalName > x( 3, SomeLogicalName( 2 ) );

Upvotes: 4

R. Martinho Fernandes
R. Martinho Fernandes

Reputation: 234394

To destroy dynamically allocated arrays you must use delete[], not delete.

While that will make the code work, it is a suboptimal solution. The following program does the same thing, but is much more succint, and safer.

#include <vector>

int main ()
{
  std::vector<std::vector<int>> x(3, std::vector<int>(2));
}

Upvotes: 10

Aesthete
Aesthete

Reputation: 18850

Every new needs a delete

Every new [] needs a delete []

Upvotes: 0

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