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;
?
Simple rules to follow:
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.
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;
Your deletion should mirror your allocation.
Since you used new [] to allocate the outer array, and new [] (in a loop) to allocate the inner arrays, do likewise for deletion. That is: your second solution is correct; delete [] the inner arrays in a loop, and finally the outer array via delete [] also.
That said, a (much, much) better solution in C++ would be to use a nested std::vector:
// Declaration and initialization:
vector<vector<double> > desc(size_out, vector<double>(size_in));
// No deletion!
Solution 2 is the right one : each cell points to a dynamically allocated array that should be deleted using delete[]. Finally,
the desc array itself should be deleted using delete[].
Bonus solution 4 : avoid using arrays and switch to std::vector<std::vector<double> >.
I would do
for (int i=0; i<size_out; i++)
delete [] desc[i];
delete [] desc;
for each array allocated with new [], you have a corresponding delete [].
And as Rupdolph says: stop using C-arrays, and start using std::vector. You will have less bugs (hundred times less bugs).
