Creating Concept Checking Classes

As an example of how to create a concept checking class template, we look at how to create the corresponding checks for the InputIterator concept. The complete definition is here:

template <class X>
struct InputIterator
  : Assignable<X>, EqualityComparable<X>
{
 private:
    typedef std::iterator_traits<X> t;
 public:
    typedef typename t::value_type value_type;
    typedef typename t::difference_type difference_type;
    typedef typename t::reference reference;
    typedef typename t::pointer pointer;
    typedef typename t::iterator_category iterator_category;

    BOOST_CONCEPT_ASSERT((SignedInteger<difference_type>));
    BOOST_CONCEPT_ASSERT((Convertible<iterator_category, std::input_iterator_tag>));
        
    BOOST_CONCEPT_USAGE(InputIterator)
    {
        X j(i);             // require copy construction
        same_type(*i++,v);  // require postincrement-dereference returning value_type
        X& x = ++j;         // require preincrement returning X&
    }
    
 private:
    X i;
    value_type v;

    // Type deduction will fail unless the arguments have the same type.
    template <typename T>
    void same_type(T const&, T const&);
};

Walkthrough

First, as a convention we name the concept checking class after the concept. Next, since InputIterator is a refinement of Assignable and EqualityComparable, we derive its concept checking class from the checking classes for those other concepts. The library will automatically check for conformance to Assignable and EqualityComparable whenever it checks the InputIterator concept.

Next, we declare the concept's associated types as member typedefs. The associated difference type is required to be a signed integer, and the iterator category has to be convertible to std::input_iterator_tag, so we assert those relationships. The syntax for accessing associated types through the concept-checking template mirrors the proposed syntax for associated type access in C++0x Finally, we use the BOOST_CONCEPT_USAGE macro to declare the function that exercises all the concept's valid expressions. Note that at this point you may sometimes need to be a little creative: for example, to check that *i++ returns the iterator's value type, we pass both values to the same_type member function template, which requires both arguments to have the same type, modulo references and cv-qualification. It's an imperfect check, but it's better than nothing.

Values for Usage Patterns Should Be Data Members

You may be wondering why we declared i and v as data members in the example above. Why didn't we simply write the following?

BOOST_CONCEPT_USAGE(InputIterator)
{
    X i;                // create the values we need
    value_type v;

    X j(i);             // require copy construction
    same_type(*i++,v);  // require postincrement-dereference returning value_type
    X& x = ++j;         // require preincrement returning X&
}

Unfortunately, that code wouldn't have worked out so well, because it unintentionally imposes the requirement that X and its value type are both default-constructible. On the other hand, since instances of the InputIterator template will never be constructed, the compiler never has to check how its data members will be constructed (C++ Standard Section 14.7.1 9). For that reason you should always declare values needed for usage patterns as data members.

These sorts of errors in concept definitions can be detected by the use of Concept Archetypes, but it's always better to avoid them pre-emptively.

Similarity to Proposed C++0x Language Support for Concepts

This library's syntaxes for concept refinement and for access of associated types mirrors the corresponding proposed syntaxes in C++0x. However, C++0x will use “signatures” rather than usage patterns to describe the valid operations on types participating in a concept, so when converting your concept checking classes into language-supported concepts, you'll need to translate your usage function into a series of signatures.

Next: Concept Covering and Archetypes
Prev: Using Concept Checks