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<Head>
<Title>Concept Checking Implementation</Title>
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<BR Clear>
<h2><a name="implementation">Implementation</a></h2>
Ideally we would like to catch, and indicate, the concept violation at
the point of instantiation. As mentioned in D&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:
<pre>
template <class RandomAccessIterator>
void stable_sort_constraints(RandomAccessIterator i)
{
typename std::iterator_traits<RandomAccessIterator>
::difference_type n;
i += n; // exercise the requirements for RandomAccessIterator
...
}
template <class RandomAccessIterator>
void stable_sort(RandomAccessIterator first, RandomAccessIterator last)
{
typedef void (*fptr_type)(RandomAccessIterator);
fptr_type x = &stable_sort_constraints;
...
}
</pre>
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>contraints()</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.
<pre>
template <class Iter>
struct RandomAccessIterator_concept
{
void constraints()
{
i += n;
...
}
typename std::iterator_traits<RandomAccessIterator>
::difference_type n;
Iter i;
...
};
</pre>
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.sgi.com/tech/stl/RandomAccessIterator.html">
RandomAccessIterator</a>.
<pre>
template <class Iter>
void stable_sort(Iter first, Iter last)
{
function_requires< RandomAccessIteratorConcept<Iter> >();
...
}
</pre>
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.
<pre>
template <class Concept>
void function_requires()
{
void (Concept::*x)() = BOOST_FPTR Concept::constraints;
ignore_unused_variable_warning(x);
}
</pre>
To check the type parameters of class templates, we provide the
<tt>BOOST_CLASS_REQUIRES</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.
<pre>
#define BOOST_CLASS_REQUIRES(type_var, concept) \
typedef void (concept <type_var>::* func##type_var##concept)(); \
template <func##type_var##concept _Tp1> \
struct concept_checking_##type_var##concept { }; \
typedef concept_checking_##type_var##concept< \
BOOST_FPTR concept <type_var>::constraints> \
concept_checking_typedef_##type_var##concept
</pre>
In addition, there are versions of <tt>BOOST_CLASS_REQUIRES</tt> that
take more arguments, to handle concepts that include interactions
between two or more types. <tt>BOOST_CLASS_REQUIRES</tt> was not used
in the implementation of the BCCL concept checks because several
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 <class Concept>
class class_requires
{
typedef void (Concept::* function_pointer)();
template <function_pointer Fptr>
struct dummy_struct { };
public:
typedef dummy_struct< BOOST_FPTR Concept::constraints > 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>
<a href="./reference.htm">Next: Reference</a><br>
<a href="prog_with_concepts.htm">Prev: Programming With Concepts</a>
<br>
<HR>
<TABLE>
<TR valign=top>
<TD nowrap>Copyright © 2000</TD><TD>
<A HREF="../../people/jeremy_siek.htm">Jeremy Siek</A>,
Univ.of Notre Dame (<A
HREF="mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</A>)
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