[lispm]

UNWIND-PROTECT is a building block.

Resource management in the MIT Lisp OS ca. 1980, approximate example.

One would define a resource of arrays, where arrays can be allocated and deallocated. They will be managed via a pool.

    (defresource 2d-array (rows columns)
      :constructor (make-array (list rows columns)))

Now user code would use the USING-RESOURCE macro, where it spans a dynamic scope. Entering the scope allocates the resource. Inside the scope the resource is allocated. Leaving the scope will automatically deallocate the resource and put it back into the pool. A deinitializer may free memory as needed.

    (using-resource (my-array 2d-array 100 100) ; get me a 100x100 array from a pool
      (setf (aref my-array 42 42) 'the-answer)  ; setting the array
      (print (aref my-array 42 42)))            ; reading the array

To make sure that the resource gets deallocated, the above macro form will expand to something using UNWIND-PROTECT:

    ...
     (unwind-protect                                            

       (progn                                                   ; protected form
         (setf my-array (allocate-resource '2d-array 100 100))  ;   allocate the array
         (setf (aref my-array 42 42) 'the-answer)               ;   setting the array
         (print (aref my-array 42 42)))                         ;  reading the array

       (when my-array                                           ; exit form
         (deallocate-resource '2d-array my-array)))             ;   deallocate the array
    ...

This is an example of manual memory management using a pool of resources, where the DEALLOCATE is done always via UNWIND-PROTECT.

Thus the user will not explicitly use UNWIND-PROTECT, but some macros which use it in their expansion...

[adrian_b]

Thanks for the explanation.

I agree that this is pretty much equivalent to RAII.

Nevertheless, it is also obvious that this was not a source of inspiration for Bjarne Stroustrup.

He has started directly from the constructors and destructors of C.A.R. Hoare (1965-11) and Simula 67 (1968-05, Kristen Nygaard & Ole-Johan Dahl).

The only change is that in 1980 he has enhanced his compiler for "C with Classes" to generate automatically all the invocations to the appropriate destructors in all the block and function epilogues, relieving the programmer from this task.