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  <title>Concept Checking Implementation</title>
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  <h2><a name="warning" id="warning"><font color=
  "red">Warning</font></a></h2>

  <p><font color="red">This documentation is out-of-date; similar but
  newer implementation techniques are now used.  This documentation
  also refers to components and protocols in the library's old
  interface such as <code>BOOST_CLASS_REQUIRES</code>
  and <code>constraints()</code> functions, which are still supported
  but deprecated.</font></p>

  <h2><a name="implementation" id="implementation">Implementation</a></h2>

  <p>Ideally we would like to catch, and indicate, the concept violation at
  the point of instantiation. As mentioned in D&amp;E[<a href=
  "bibliography.htm#stroustrup94:_design_evolution">2</a>], the error can be
  caught by exercising all of the requirements needed by the function
  template. Exactly how the requirements (the valid expressions in
  particular) are exercised is a tricky issue, since we want the code to be
  compiled—<i>but not executed</i>. Our approach is to exercise the
  requirements in a separate function that is assigned to a function pointer.
  In this case, the compiler will instantiate the function but will not
  actually invoke it. In addition, an optimizing compiler will remove the
  pointer assignment as ``dead code'' (though the run-time overhead added by
  the assignment would be trivial in any case). It might be conceivable for a
  compiler to skip the semantic analysis and compilation of the constraints
  function in the first place, which would make our function pointer
  technique ineffective. However, this is unlikely because removal of
  unnecessary code and functions is typically done in later stages of a
  compiler. We have successfully used the function pointer technique with GNU
  C++, Microsoft Visual C++, and several EDG-based compilers (KAI C++, SGI
  MIPSpro). The following code shows how this technique can be applied to the
  <tt>std::stable_sort()</tt> function:</p>
  <pre>
  template &lt;class RandomAccessIterator&gt;
  void stable_sort_constraints(RandomAccessIterator i)
  {
    typename std::iterator_traits&lt;RandomAccessIterator&gt;
      ::difference_type n;
    i += n;  // exercise the requirements for RandomAccessIterator
    ...
  }
  template &lt;class RandomAccessIterator&gt;
  void stable_sort(RandomAccessIterator first, RandomAccessIterator last)
  {
    typedef void (*fptr_type)(RandomAccessIterator);
    fptr_type x = &amp;stable_sort_constraints;
    ...
  }
</pre>

  <p>There is often a large set of requirements that need to be checked, and
  it would be cumbersome for the library implementor to write constraint
  functions like <tt>stable_sort_constraints()</tt> for every public
  function. Instead, we group sets of valid expressions together, according
  to the definitions of the corresponding concepts. For each concept we
  define a concept checking class template where the template parameter is
  for the type to be checked. The class contains a <tt>constraints()</tt>
  member function which exercises all of the valid expressions of the
  concept. The objects used in the constraints function, such as <tt>n</tt>
  and <tt>i</tt>, are declared as data members of the concept checking
  class.</p>
  <pre>
  template &lt;class Iter&gt;
  struct RandomAccessIteratorConcept
  {
    void constraints()
    {
      i += n;
      ...
    }
    typename std::iterator_traits&lt;RandomAccessIterator&gt;
      ::difference_type n;
    Iter i;
    ...
  };
</pre>

  <p>We can still use the function pointer mechanism to cause instantiation
  of the constraints function, however now it will be a member function
  pointer. To make it easy for the library implementor to invoke the concept
  checks, we wrap the member function pointer mechanism in a function named
  <tt>function_requires()</tt>. The following code snippet shows how to use
  <tt>function_requires()</tt> to make sure that the iterator is a <a href=
  "http://www.boost.org/sgi/stl/RandomAccessIterator.html">RandomAccessIterator</a>.</p>
  <pre>
  template &lt;class Iter&gt;
  void stable_sort(Iter first, Iter last)
  {
    function_requires&lt; RandomAccessIteratorConcept&lt;Iter&gt; &gt;();
    ...
  }
</pre>

  <p>The definition of the <tt>function_requires()</tt> is as follows. The
  <tt>Concept</tt> is the concept checking class that has been instantiated
  with the modeling type. We assign the address of the constraints member
  function to the function pointer <tt>x</tt>, which causes the instantiation
  of the constraints function and checking of the concept's valid
  expressions. We then assign <tt>x</tt> to <tt>x</tt> to avoid unused
  variable compiler warnings, and wrap everything in a do-while loop to
  prevent name collisions.</p>
  <pre>
  template &lt;class Concept&gt;
  void function_requires()
  {
    void (Concept::*x)() = BOOST_FPTR Concept::constraints;
    ignore_unused_variable_warning(x);
  }
</pre>

  <p>To check the type parameters of class templates, we provide the
  <tt>BOOST_CLASS_REQUIRE</tt> macro which can be used inside the body of a
  class definition (whereas <tt>function_requires()</tt> can only be used
  inside of a function body). This macro declares a nested class template,
  where the template parameter is a function pointer. We then use the nested
  class type in a typedef with the function pointer type of the constraint
  function as the template argument. We use the <tt>type_var</tt> and
  <tt>concept</tt> names in the nested class and typedef names to help
  prevent name collisions.</p>
  <pre>
#define BOOST_CLASS_REQUIRE(type_var, ns, concept) \
  typedef void (ns::concept &lt;type_var&gt;::* func##type_var##concept)(); \
  template &lt;func##type_var##concept _Tp1&gt; \
  struct concept_checking_##type_var##concept { }; \
  typedef concept_checking_##type_var##concept&lt; \
    BOOST_FPTR ns::concept&lt;type_var&gt;::constraints&gt; \
    concept_checking_typedef_##type_var##concept
</pre>

  <p>In addition, there are versions of <tt>BOOST_CLASS_REQUIRE</tt> that
  take more arguments, to handle concepts that include interactions between
  two or more types. <tt>BOOST_CLASS_REQUIRE</tt> was not used in the
  implementation of the BCCL concept checks because some compilers do not
  implement template parameters of function pointer type. 
  <!-- We decided not to go with this version since it is easier to misuse

To check the type parameters of class templates, we provide the
<tt>class_requires</tt> class which can be used inside the body of a
class definition (whereas <tt>function_requires()</tt> can only be
used inside of a function body).  <tt>class_requires</tt> declares a
nested class template, where the template parameter is a function
pointer. We then use the nested class type in a typedef with the
function pointer type of the constraint function as the template
argument.

<pre>
  template &lt;class Concept&gt;
  class class_requires
  {
    typedef void (Concept::* function_pointer)();

    template &lt;function_pointer Fptr&gt;
    struct dummy_struct { };
  public:
    typedef dummy_struct&lt; BOOST_FPTR Concept::constraints &gt; check;
  };
</pre>

<tt>class_requires</tt> was not used in the implementation of the
Boost Concept Checking Library concept checks because several
compilers do not implement template parameters of function pointer
type.

--></p>

  <p><a href="./reference.htm">Next: Reference</a><br />
  <a href="prog_with_concepts.htm">Prev: Programming With
  Concepts</a><br /></p>
  <hr />

  <table>
    <tr valign="top">
      <td nowrap="nowrap">Copyright &copy; 2000</td>

      <td><a href="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</a>(<a href=
      "mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>) Andrew
      Lumsdaine(<a href="mailto:lums@osl.iu.edu">lums@osl.iu.edu</a>),
        2007 <a href="mailto:dave@boost-consulting.com">David Abrahams</a>.
    </tr>
  </table>
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