Passing unique_ptr to library functions (FORTRAN function)

Question

I am using LAPACK library to create a R-package using C++. I am using unique_ptr for defining the arrays as

   unique_ptr<double[]> my_arr(new double[arr_length]);

I then pass this unique_ptr to library function (FORTRAN function) which accepts pointer to double array and will update this array inside the function as

   F77_CALL(daxpy) (&num_feat_, &beta, tmp, &inc_one, my_arr.get(), &inc_one);

After going through web, I noticed it is not recommended to pass unique_ptr as pointer argument to a function. However, the library functions I am using needs a pointer in their argument. I can not release the pointer before sending it to the function since the library function needs to update the pointer. Is there any efficient way to handle this?

Answer 1

Assuming the library is not going to take ownership of the array and try to delete it itself I think this is perfectly fine.

You should generally prefer to pass by reference or raw-pointer and normally only pass unique_ptr when you are transferring ownership so I think this is correct. The calling code retains unique ownership of the array.

You know the array will not be deleted until after the function call when the unique_ptr goes out-of-scope which is exactly what you want.

I think this is the right way of calling functions that are not going to take ownership even if they are in your own code.

See GotW #91 for a summary on how to pass (smart) pointers.

If the library retained the pointer after the function call you would have to ensure the unique_ptr did not go out-of-scope before the library finished using it which is a little more tricky.

There are some libraries that assume you will allocate objects on the heap and give them ownership via a raw pointer (I've seen it in some visualization libraries). They delete the object when they are done. This is generally considered bad practice in C++11 but if you need to call such libraries you should not use a unique_ptr as you don't want to delete the object yourself.