Databases
StackOverflow2013
Postsdbo
POConst-correctness and immutable allocated objects
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POConst-correctness and immutable allocated objects
    primarykey
    data
    text
    <p>During a recent discussion (see comments to <a href="https://stackoverflow.com/questions/3965279/opaque-c-structs-how-should-they-be-declared/3965308#3965308">this answer</a>), R.. recommended to never create aliases for pointer-to-<code>const</code> types as you won't be able to deallocate the referenced objects easily in a conforming C program (remember: <code>free()</code> takes a non-<code>const</code> pointer argument <del>and C99 6.3.2.3 only allows conversions from non-qualified to qualified</del>).</p> <p>The C language obviously assumes the existance of an owner for any allocated object, ie someone somewhere has to store a non-<code>const</code> pointer to the object, and this someone is responsible for deallocation.</p> <p>Now, consider a library allocating and initializing objects which are non-modifyable from user-space code, so function calls always return <code>const</code>-qualified pointers.</p> <p>Clearly, the library is the owner of the object and should retain a non-<code>const</code> pointer, which is somewhat silly as the user already supplies a perfectly valid, but <code>const</code> copy of the pointer on each library call.</p> <p>To deallocate such an object, the library has to discard the <code>const</code> qualifier; as far as I can tell, the following</p> <pre><code>void dealloc_foo(const struct foo *foo) { free((void *)foo); } </code></pre> <p>is valid C; it would only be invalid if the <code>foo</code> parameter were additionally <code>restrict</code>-qualified.</p> <p>However, casting away <code>const</code> seems somewhat hack-ish.</p> <p>Is there another way aside from dropping the <code>const</code> from all return values of library functions, which would lose any information about object mutability?</p>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USCommunity
    1. USChristoph
    plurals
    1. PL
      singulars
      1. LTLinked
    1. PL
      singulars
      1. LTLinked
    2. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POConst-correctness and immutable allocated objects
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POConst-correctness and immutable allocated objects
      1. This table or related slice is empty.
      1. VTUpMod
    1. COSee my answer. This use of `const` is simply wrong.
      singulars
      1. POConst-correctness and immutable allocated objects
      1. USR..
    2. COIf you hadn't followed the bad advice in that other question, you wouldn't be worried about some other code mucking around inside your data structure to the point you felt compelled to apply `const`. The correct level of abstraction for a handle is that your library, and only your library, can use it to find the associated object internal data. Users of the library **should not know** whether the handle is a memory address (aka pointer), index into one of your data structures, or some other key, just that it is a value produced and used by your library and meaningful only in that context.
      singulars
      1. POConst-correctness and immutable allocated objects
      1. USBen Voigt
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload