A double pointer has two basic meanings.
One is of a pointer to a pointer, where changing the value of double pointer will result in the original pointer being changed.
Another is that of a two-dimensional array, such as a matrix
Example for Two dimensional array
int main( int argc, char **argv (or) char* argv[] )
{
// print the first string
printf("%s\n", argv[0]);
return 0;
}
The other example where the double pointer is a pointer to a pointer, lets say you have a function foo() that allocates memory for a pointer that was declared in function main()
void foo( char ** ptr)
{
*ptr = malloc(255); // allocate some memory
strcpy( *ptr, "Hello World");
}
int main()
{
char *ptr = 0;
// call function with a pointer to pointer
foo( &ptr );
printf("%s\n", ptr);
// free up the memory
free(ptr);
return 0;
}
Creating 2D int array
//Create your pointer
int **ptr;
//Assign first dimension
ptr = new int*[5];
//Assign second dimension
for(int i = 0; i < 5; i++)
ptr[i] = new int[5];
Deleting 2D array
for (int x = 0; x < nCol; ++x)
delete [] ptr[x];
delete [] ptr;
How delete[] knows how many items to be destroyed ??
When an array is created with new[], the size of array is stored in the metadata at the memory block. delete[] makes use of that information.
New[] operator requires default constructor , for ex
student* kar = new student[10](34,""); // its completely wrong
student * kar = new student[10]; // Correct
new & delete are based on malloc & free. Malloc additionally allocate a header storing the size allocated, so free() knows the size to free. new[] not only keeps the malloc header, but also has an extra header storing the size of the array/vector, so delete[] knows how many objects are in the array/vector, and thus knows how many times to call the destructor.
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Allocation Using
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Deallocation using
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Result
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Remarks
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new
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delete
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Ok
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new[]
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delete[]
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Ok
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new
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delete[]
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The vector delete
will try to delete more objects depending on the random value it gets from
its object-count location and will obviously result in heap corruption.
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Compiler will not
track the Pointer, Its developer responsibility to call the respective delete
operator (ie delete or delete[])
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new[]
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delete
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Memory leak
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It will delete only
first item in the array
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