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  1. <html>
  2.   <head>
  3.     <title>Class deletions</title>
  4.   </head>
  5.   <body>
  6.     <h1>Class deletions</h1>
  7.     <table>
  8.       <tr>
  9.         <td>Document number:</td>
  10.         <td><i>Nnnnn=yy&ndash;nnnn</i></td>
  11.       </tr>
  12.       <tr>
  13.         <td>Date:</td>
  14.         <td><i>2014&ndash;05&ndash;28</i></td>
  15.       </tr>
  16.       <tr>
  17.         <td>Project:</td>
  18.         <td>Programming Language C++, Evolutionary Working Group</td>
  19.       </tr>
  20.       <tr>
  21.         <td>Reply-to:</td>
  22.         <td><a href="mailto:[email protected]">Douglas Boffey
  23.              &lt;[email protected]></a></td>
  24.       </tr>
  25.     </table>
  26.     <h2>Table of Contents</h2>
  27.       <a href="#intro">Introduction</a><p/>
  28.       <a href="#motivation">Motivation and Scope</a><p/>
  29.       <a href="#impact">Impact on the Standard</a><p/>
  30.       <a href="#design">Design decisions</a><p/>
  31.       <a href="#technical">Technical Specifications</a><p/>
  32.       <a href="#ack">Acknowledgements</a><p/>
  33.       <a href="#refs">References</a><p/>
  34.       <a href="#change">Change Log</a><p/>
  35.  
  36.     <h2><a id="intro"/>Introduction</h2>
  37.       This proposal is for the introduction of deleted classes, allowing the following class definition:<p/>
  38.       <code>class Foo = delete;</code><p/>
  39.       This would make any further reference to class Foo an error.<p/>
  40.  
  41.     <h2><a id="motivation"/>Motivation and Scope</h2>
  42.       There are two problems this proposal addresses:<p/>
  43.       <ol>
  44.         <li><i>templates that are not substantiable</i>&mdash;sometimes, you may need to create a
  45.             template class, but only specialisations have any meaning [EXAMPLE: consider the
  46.             template class std::numeric_limits<T>.  This only makes sense if T is an arithmetic
  47.             type.  A design decision was made to use default values if T is not an arithmetic
  48.             type.  An alternative design decision would be to leave the general template
  49.             unsubstantiable.  The best way to achieve this would be to delete the class.
  50.             &ndash; &ndash;END EXAMPLE].<p/>
  51.             Alternatively, a specialisation of a group of specialisations needs to be
  52.             deleted [EXAMPLE: in Chapter 29 of <a href="#stro2013">[STRO2013]</a>, a
  53.             Matrix class is developed that is templated on the cell type T, and the number
  54.             of dimensions, N, of the matrix.<p/>
  55.             Matrix&lt;T,0> could have been defined equivalent to a scalar, but, on p.&nbsp;844,
  56.             we read:<p/>
  57.             To avoid surprises, we define N=0 to be an error:<p/>
  58.             <code><b>template</b>&lt;<b>typename</b> T><p/>
  59.                   <b>struct</b> Matrix_init&lt;T,0>; <i>// undefined on purpose</i></code><p/>
  60.             It would surely be preferable to explicitly prevent it being defined by deleting
  61.             it&ndash; &ndash;END EXAMPLE]; and</li>
  62.         <li><i>deprecated classes</i>&mdash;after a class has been deprecated, it may
  63.             no longer be supported.  This is best done by deleting it.</li>
  64.       </ol>
  65.       The notation was chosen to mirror the way functions can be deleted at the
  66.          moment.<p/>
  67.       One shortcoming is that there would be no method of indicating the reason for the
  68.          deletion, except within documentation.<p/>
  69.  
  70.     <h2><a id="impact"/>Impact on the Standard</h2>
  71.       This would allow a class definition to indicate that the class was deleted.<p/>
  72.  
  73.     <h2><a id="design"/>Design Decisions</h2>
  74.       There are currently two methods of achieving this:<p/>
  75.       <ol>
  76.         <li>not creating a class definition&mdash;this would leave the programmer to
  77.             write his/her own, this negates the purpose of not defining the class; and</li>
  78.         <li>adding a <code>std::static_assert</code> to the body of the class&mdash;this
  79.             has the following problems:</li>
  80.           <ul>
  81.             <li>having a static_assert seems to be abusing the r&ocirc;le of a static_assert, and</li>
  82.             <li>code to indicate a class is not to be substantiated should be part of the
  83.                 header, not the body of the definition.</li>
  84.           </ul>
  85.       </ol>
  86.       This could alternatively be solved by adding an attribute, for example, [[poison]],
  87.         but this would be changing the standard in a way that is contrary to the current language.<p/>
  88.  
  89.       To mirror what is done with deleted functions, and to lessen the burden on compiler writers,
  90.         it was decided to allow deleted classes to be pointed to and referenced.  Since it is
  91.         meaningless to speak of the size of a nonexistant object, it was decided to disallow taking
  92.         the sizeof, and of using pointer arithmetic, on deleted classes<p/>
  93.  
  94.       As there would be complications in having a deleted class participate in a class hierarchy,
  95.         it was decided not to allow deleted classes to have base classes, nor be derived from.
  96.  
  97.     <h2><a id="technical"/>Technical specifications</h2>
  98.       A class deletion would define a class that could not be used.  It would have the following
  99.         properties:<p/>
  100.       <ul>
  101.         <li>Deleted classes can be declared any number of times, but only defined once (as per the ODR).
  102.           [EXAMPLE:
  103.           <code>
  104.             <b>class</b> A;<br/>
  105.             <br/>
  106.             <b>class</b> A = <b>delete</b>; <i>// ok</i><br/>
  107.             <br/>
  108.             <b>class</b> A; <i>// ok</i><br/>
  109.             <br/>
  110.             <b>class</b> A {}; <i>// error: class deleted</i><br/>
  111.             <br/>
  112.             <b>class</b> A = <b>delete</b>; <i>// error: violation of ODR</i><br/>
  113.           </code> &ndash; &ndash;END EXAMPLE]</li>
  114.         <li>Deleted classes can not have base classes, nor can they be derived from.  [EXAMPLE:
  115.           <code>
  116.             <b>class</b> A {};<br/>
  117.             <br/>
  118.             <b>class</b> B : A = <b>delete</b>; <i>// error: deleted classes cannot have a base class</i><br/>
  119.             <br/>
  120.             <b>class</b> C : B; <i>// error: class B deleted</i><br/>
  121.           </code> &ndash; &ndash;END EXAMPLE]</li>
  122.         <li>Deleted classes cannot be substantiated but can be pointed to and referenced.  [EXAMPLE:
  123.           <code>
  124.             <b>class</b> A;<br/>
  125.             <br/>
  126.             A *a;<br/>
  127.             <br/>
  128.             <b>class</b> A = <b>delete</b>; <i>// ok</i><br/>
  129.             <br/>
  130.             <b>class</b> B;<br/>
  131.             <br/>
  132.             <b>class</b> C {<br/>
  133.             &nbsp;&nbsp;B *b;<br/>
  134.             };<br/>
  135.             <br/>
  136.             <b>class</b> B = <b>delete</b>; <i>// ok</i><br/>
  137.             <br/>
  138.             <b>class</b> D = <b>delete</b>;<br/>
  139.             <br/>
  140.             D *d; <i>// ok</i><br/>
  141.             D d; <i>// error: D deleted</i><br/>
  142.             <b>void</b> fn1(D d); <i>// error: D deleted</i><br/>
  143.             <b>void</b> fn2(D *d); <i>// ok</i><br/>
  144.             <b>void</b> fn3(D &d); <i>// ok</i><br/>
  145.             <b>void</b> fn4(D &&d); <i>// ok</i><br/>
  146.             D fn5(); <i>// error: D deleted</i><br/>
  147.             D *fn6(); <i>// ok</i><br/>
  148.             D &fn7(); <i>// ok</i><br/>
  149.             D &&fn8(); <i>// ok</i><br/>
  150.           </code> &ndash; &ndash;END EXAMPLE]</li>
  151.         <li>Deleted classes can be befriended.  [EXAMPLE:
  152.           <code>
  153.             <b>class</b> A = <b>delete</b>;<br/>
  154.             <br/>
  155.             <b>class</b> B {<br/>
  156.             &nbsp;&nbsp;<b>friend</b> <b>class</b> A; <i>// ok</i><br/>
  157.             &nbsp;&nbsp;<b>friend</b> <b>class</b> C;<br/>
  158.             };<br/>
  159.             <br/>
  160.             <b>class</b> C = <b>delete</b>; <i>// ok</i><br/>
  161.           </code> &ndash; &ndash;END EXAMPLE]</li>
  162.         <li>Deleted classes can be aliased. [EXAMPLE:
  163.           <code>
  164.             <b>class</b> A = <b>delete</b>;<br/>
  165.             <br/>
  166.             <b>typedef</b> A B; <i>// ok: B treated as deleted</i><br/>
  167.             <b>using</b> C = A; <i>// ok: C treated as deleted</i><br/>
  168.             <br/>
  169.             B *b; <i>// error: A deleted</i><br/>
  170.             C *c; <i>// error: A deleted</i><br/>
  171.           </code> &ndash; &ndash;END EXAMPLE]</li>
  172.         <li>sizeof cannot be called on a deleted class, but typeid can be.  operator new is
  173.           not allowed to return a deleted class.  No pointer arithmetic is allowed on deleted
  174.           classes.  [EXAMPLE:
  175.           <code>
  176.             <b>class</b> A = deleted;<br/>
  177.             <b>auto</b> s{<b>sizeof</b>(A)}; <i>// error: A deleted</i><br/>
  178.             <b>auto</b> t{<b>typeid</b>(A)}; <i>// ok</i><br/>
  179.             <br/>
  180.             <b>auto</b> b = <b>new</b> A; <i>// error: A deleted</i><br/>
  181.             <b>auto</b> c = <b>new</b> A[10]; <i>// error: A deleted</i><br/>
  182.             <b>auto</b> d = <b>new</b> (/* whatever */) A; <i>// error: A deleted</i><br/>
  183.             <br/>
  184.             A *p;<br/>
  185.             A *q;<br/>
  186.             <br/>
  187.             ++p; <i>// error: A deleted</i><br/>
  188.             q = p++; <i>// error: A deleted</i><br/>
  189.             --p; <i>// error: A deleted</i><br/>
  190.             q = p--; <i>// error: A deleted</i><br/>
  191.             q = p + 2; <i>// error: A deleted</i><br/>
  192.             p += 2; <i>// error: A deleted</i><br/>
  193.             q = p - 2; <i>// error: A deleted</i><br/>
  194.             p -= 2; <i>// error: A deleted</i><br/>
  195.             std::ptrdiff r = p - q; <i>// error: A deleted</i><br/>
  196.           </code> &ndash; &ndash;END EXAMPLE]</li>
  197.         <li>Similarly, casting to a pointer to a deleted class is an error.  [EXAMPLE:
  198.           <code>
  199.           </code> &ndash; &ndash;END EXAMPLE]</li>
  200.       </ul>
  201.  
  202.     <h2><a id="ack"/>Acknowledgements</h2>
  203.       I would like to give thanks to everyone who has provided advice on the proposal website.<p/>
  204.  
  205.     <h2><a id="refs"/>References</h2>
  206.     <a id="#stro2013"/><b>[STRO2013]</b> Stroustrup, B. <i>The C++ Programming
  207.        Language, 4<sup>th</sup> edition</i> Addison-Wesley, 2013, p. 844<p/>
  208.  
  209.     <h2><a id="change"/>Change Log</h2>
  210.     <ul>
  211.       <li>2014&ndash;06&ndash;05
  212.         <ul>
  213.           <li>added this change log</li>
  214.           <li>added title</li>
  215.           <li>removed all inappropriate &lt;code> tags</li>
  216.           <li>removed the &lsquo;unconditional’ from the static_assert</li>
  217.           <li>made a first draft at the technical specifications</li>
  218.           <li>changed the format of the references</li>
  219.         </ul>
  220.       </li>
  221.       <li>2014&ndash;06&ndash;06
  222.         <ul>corrected &lt;p> tags</ul>
  223.         <ul>changed rule for classes derived from a deleted class</ul>
  224.         <ul>added rationale for not allowing pointers to deleted classes, using sizeof on them</ul>
  225.         <ul>added rules for sizeof, typeid, operator new and *_cast</ul>
  226.       </li>
  227.       <li>2014&ndash;06&ndash;10
  228.         <ul>improved example of general template with deleted specialisation</ul>
  229.         <ul>added example of deleted general template with substantiated specialisations</ul>
  230.         <ul>the <a href="motivation">motivation</a> was updated to reflect revised rules</ul>
  231.         <ul>changed the examples in the <a href="technical">Technical Specification</a>
  232.           to reflect the change in rules</ul>
  233.       </li>
  234.     </ul>
  235.   </body>
  236. </html>
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