Delete a pointer to pointer (as array of arrays)

Question

I have this in my code:

double** desc = new double* [size_out]; for (int i = 0; i < size_out; i++)     desc[i] = new double [size_in]; 

How do I delete this desc?

Should I do:

delete [] desc; 

or

for (int i=0; i<size_out; i++)     delete [] desc[i]; delete [] desc; 

or

for (int i=0; i<size_out; i++)     delete [] desc[i]; delete desc; 

?

Answer 1

Simple rules to follow:

• for each allocation, there has to be a deallocation (ex1 is therefore wrong)
• what was allocated using new should be freed using delete, using new[] should be deallocated using delete[] and using malloc should be deallocated using free (ex3 is therefore wrong)

Conclusion, ex2 is OK.

Answer 2

Your code shouldn't compile. The type of an array new expression is a pointer to the type of array element being created (the value is a pointer to the first element of the allocated array).

So the type of new double**[size_out] is double ***.

Whenever you use the array form of new, you must use the array form of delete even if you only allocate an array of size one.

double*** desc = new double**[size_out]; for (int i=0; i<size_out; i++)     desc[i] = new double*[size_in];   for (int i=0; i<size_out; i++)     delete[] desc[i];  delete[] desc; 

Note that you still haven't allocated any double, just pointers.

Did you really want this instead?

double** desc = new double*[size_out]; for (int i=0; i<size_out; i++)     desc[i] = new double[size_in];  for (int i=0; i<size_out; i++)     delete[] desc[i];  delete[] desc;