initialization of double pointer with new

Hi!

Can anyone, please, write me an example of initialization of double pointer with new?

char **tableOfStrings = new ...???

Thanks!

You would do something like this:

Code:

---

#include <iostream>

int main(void)
{
  int np = 10;
  int *p;
  int **dp;
 
  p = new int;
  dp = new int *;
 
  if(p == NULL || dp == NULL)
  {
    std::cout<<"Could Not Allocate Memory"<<std::endl;
    std::cin.get();
    return 0;
  } 
 
  p = &np;
  dp = &p;
 
  std::cout<<np<<std::endl;
  std::cout<<*p <<std::endl;
  std::cout<<**dp<<std::endl;
     
  **dp = 15;
 
  std::cout<<np<<std::endl;
  std::cout<<*p <<std::endl;
  std::cout<<**dp<<std::endl;
   
  delete p;
  delete dp;

  std::cin.get();
  return 0;
 
}

---

delete p;
delete dp;

Except you trashed your allocated pointers with
p = &np;
dp = &p;

>Can anyone, please, write me an example of initialization of double pointer with new?

Code:

---

const int len = 80;
char **tableOfStrings = new char[10];
for (int i=0; i<10; i++)
  tableOfStrings[i] = new char[len];

---

Swoopy: You mean,

Code:

---

char **tableOfStrings = new char*[10];
for (int i=0; i<10; i++)
  tableOfStrings[i] = new char[len];
 
//Work with your tableOfStrings
 
for(int i = 0; i < 10; i++)
  delete[] tableOfStrings[i];
delete tableOfStrings;

---

>Can anyone, please, write me an example of initialization of double pointer with new?
Lightning fast method, but don't try this at home otherwise your instructor will shoot you!

Code:

---

const int rows = 500;
const int maxSizeOfEachRow = 200;
 
char** tableOfStrings = (char**)(new char[rows * sizeOfRow]);
 
//... do something with your table of strings
 
delete[] tableOfStrings[0];

---

>Swoopy: You mean,
Yes, thanks Hunter, as one of our other members would say, I was on the phone at the time. ;)

I get a debug assertion on delete when I try that code, Hunter (using VC6). After poking at it so that the delete worked, I got access violations when I tried to use tableOfStrings[x][y] to access an individual character.

Code:

---

int main()
{
        const int rows = 500;
        const int maxSizeOfEachRow = 10;

        char** tableOfStrings = NULL;
        tableOfStrings = (char**)(new char[rows * maxSizeOfEachRow]);
 
        //... do something with your table of strings
//        tableOfStrings[0][0] = 'a';  //causes bad things to happen
        tableOfStrings[0] = "123456789";
        tableOfStrings[1] = "abcdefghi";

        cout << tableOfStrings[0] << endl;
        cout << tableOfStrings[1] << endl;
       
        delete tableOfStrings;
//        delete[] tableOfStrings[0];  //debug assertion

        return 0;
}

---

Am I doing something wrong, or should I not expect this sort of thing to allow individual character access?

> Am I doing something wrong
Actually, you're doing everything wrong.

> tableOfStrings = (char**)(new char[rows * maxSizeOfEachRow]);
If you're casting new, think that you're doing something very wrong.

In this case, you should have written

Code:

---

tableOfStrings = new char*[rows];

---

Now that's only part of the story, now allocate each row as well

Code:

---

for ( int r = 0 ; r < rows ; r++ ) tableOfStrings[r] = new char[maxSizeOfEachRow];

---

Now you can do this

Code:

---

tableOfStrings[0][0] = 'a';

---

> tableOfStrings[0] = "123456789";
Whilst this is legal, you've just leaked some memory, and set up a failure when you come to delete this memory.

Code:

---

strcpy( tableOfStrings[0], "123456789" );

---

> delete tableOfStrings;
Nested structures are freed in reverse order.

Code:

---

for ( int r = 0 ; r < rows ; r++ ) delete [] tableOfStrings[r];
delete [] tableOfStrings;

---

Quote:

---

> tableOfStrings = (char**)(new char[rows * maxSizeOfEachRow]);
If you're casting new, think that you're doing something very wrong.

---

Oops. I remember this method being posted some time ago as a faster method of allocating a 2D array, though apparently I remembered the method wrong. It should really be:

Code:

---

char** tableOfStrings = new char*[rows];
char* bigMemoryChunk = new char[rows * rowLength];
for(int i = 0; i < rows; ++i)
  tableOfStrings[i] = bigMemoryChunk + (rowLength * i);
 
...
 
delete[] bigMemoryChunk;
delete[] tableOfStrings;

---

My apologies for the confusion. And this still probably shouldn't be done unless what you're doing is time-critical anyway, which I highly doubt it is at this point. It's just something to keep at the back of your mind until the day comes when you'll actually need to use it.