Boost.Range

Utilities

Having an abstraction that encapsulates a pair of iterators is very useful. The standard library uses std::pair in some circumstances, but that class is cumbersome to use because we need to specify two template arguments, and for all range algorithm purposes we must enforce the two template arguments to be the same. Moreover, std::pair<iterator,iterator> is hardly self-documenting whereas more domain specific class names are. Therefore these two classes are provided:

The iterator_range class is templated on an Forward Traversal Iterator and should be used whenever fairly general code is needed. The sub_range class is templated on an Forward Range and it is less general, but a bit easier to use since its template argument is easier to specify. The biggest difference is, however, that a sub_range can propagate constness because it knows what a corresponding const_iterator is.

Both classes can be used as ranges since they implement the minimal interface required for this to work automatically.

Class iterator_range

The intention of the iterator_range class is to encapsulate two iterators so they fulfill the Forward Range concept. A few other functions are also provided for convenience.

If the template argument is not a model of Forward Traversal Iterator, one can still use a subset of the interface. In particular, size() requires Forward Traversal Iterators whereas empty() only requires Single Pass Iterators.

Recall that many default constructed iterators are singular and hence can only be assigned, but not compared or incremented or anything. However, if one creates a default constructed iterator_range, then one can still call all its member functions. This means that the iterator_range will still be usable in many contexts even though the iterators underneath are not.

Synopsis

namespace boost
{
    template< class ForwardTraversalIterator >
    class iterator_range
    {
    public: // Forward Range types
        typedef ...                        value_type;
        typedef ...                        difference_type;
        typedef ...                        size_type;
        typedef ForwardTraversalIterator   iterator;
        typedef ForwardTraversalIterator   const_iterator;

    public: // construction, assignment
        template< class ForwardTraversalIterator2 >
        iterator_range( ForwardTraversalIterator2 Begin, ForwardTraversalIterator2 End );
                    
        template< class ForwardRange >
        iterator_range( ForwardRange& r );
  
        template< class ForwardRange >
        iterator_range( const ForwardRange& r );
        
        template< class ForwardRange >
        iterator_range& operator=( ForwardRange& r );

        template< class ForwardRange >
        iterator_range& operator=( const ForwardRange& r );
    
    public: // Forward Range functions
        iterator  begin() const;
        iterator  end() const;
        size_type size() const;
        bool      empty() const;
        
    public: // convenience
        operator    unspecified_bool_type() const;
        bool        equal( const iterator_range& ) const;
        value_type& front() const;
        value_type& back() const;
        // for Random Access Range only: 
        value_type& operator[]( size_type at ) const;
    };
    
    // stream output
    template< class ForwardTraversalIterator, class T, class Traits >
    std::basic_ostream<T,Traits>& 
    operator<<( std::basic_ostream<T,Traits>& Os,
                const iterator_range<ForwardTraversalIterator>& r );

    // comparison
    template< class ForwardTraversalIterator, class ForwardTraversalIterator2 >
    bool operator==( const iterator_range<ForwardTraversalIterator>& l, 
                     const iterator_range<ForwardTraversalIterator2>& r );

    template< class ForwardTraversalIterator, class ForwardRange >
    bool operator==( const iterator_range<ForwardTraversalIterator>& l, 
                     const ForwardRange& r );

    template< class ForwardTraversalIterator, class ForwardRange >
    bool operator==( const ForwardRange& l,
                     const iterator_range<ForwardTraversalIterator>& r );

    template< class ForwardTraversalIterator, class ForwardTraversalIterator2 >
    bool operator!=( const iterator_range<ForwardTraversalIterator>& l, 
                     const iterator_range<ForwardTraversalIterator2>& r );

    template< class ForwardTraversalIterator, class ForwardRange >
    bool operator!=( const iterator_range<ForwardTraversalIterator>& l, 
                     const ForwardRange& r );

    template< class ForwardTraversalIterator, class ForwardRange >
    bool operator!=( const ForwardRange& l,
                     const iterator_range<ForwardTraversalIterator>& r );

    template< class ForwardTraversalIterator, class ForwardTraversalIterator2 >
    bool operator<( const iterator_range<ForwardTraversalIterator>& l, 
                    const iterator_range<ForwardTraversalIterator2>& r );

    template< class ForwardTraversalIterator, class ForwardRange >
    bool operator<( const iterator_range<ForwardTraversalIterator>& l, 
                    const ForwardRange& r );

    template< class ForwardTraversalIterator, class ForwardRange >
    bool operator<( const ForwardRange& l,
                    const iterator_range<ForwardTraversalIterator>& r );
 
    // external construction
    template< class ForwardTraversalIterator >
    iterator_range< ForwardTraversalIterator >
    make_iterator_range( ForwardTraversalIterator Begin, 
                         ForwardTraversalIterator End );
       
    template< class ForwardRange >
    iterator_range< typename iterator_of<ForwardRange>::type >
    make_iterator_range( ForwardRange& r );

    template< class ForwardRange >
    iterator_range< typename const_iterator_of<ForwardRange>::type >
    make_iterator_range( const ForwardRange& r );
    
    template< class Range >
    iterator_range< typename range_iterator<Range>::type >
    make_iterator_range( Range& r,
                         typename range_difference<Range>::type advance_begin,
                         typename range_difference<Range>::type advance_end );
    
    template< class Range >
    iterator_range< typename range_const_iterator<Range>::type >
    make_iterator_range( const Range& r, 
                         typename range_difference<Range>::type advance_begin,
                         typename range_difference<Range>::type advance_end );
    
    // convenience
    template< class Sequence, class ForwardRange >
    Sequence copy_range( const ForwardRange& r );
    
} // namespace 'boost'

If an instance of iterator_range is constructed by a client with two iterators, the client must ensure that the two iterators delimit a valid closed-open range [begin,end).

It is worth noticing that the templated constructors and assignment operators allow conversion from iterator_range<iterator> to iterator_range<const_iterator>. Similarly, since the comparison operators have two template arguments, we can compare ranges whenever the iterators are comparable; for example when we are dealing with const and non-const iterators from the same container.

Details member functions

operator unspecified_bool_type() const;

Returns !empty();

bool equal( iterator_range& r ) const;

Returns begin() == r.begin() && end() == r.end();

Details functions

bool operator==( const ForwardRange1& l, const ForwardRange2& r );

Returns size(l) != size(r) ? false : std::equal( begin(l), end(l), begin(r) );

bool operator!=( const ForwardRange1& l, const ForwardRange2& r );
Returns !( l == r );
bool operator<( const ForwardRange1& l, const ForwardRange2& r );
Returns std::lexicographical_compare( begin(l), end(l), begin(r), end(r) ); 

iterator_range make_iterator_range( Range& r, 
                                    typename range_difference<Range>::type advance_begin, 
                                    typename range_difference<Range>::type advance_end );
Effects: 
iterator new_begin = begin( r ),
iterator new_end   = end( r );
std::advance( new_begin, advance_begin );
std::advance( new_end, advance_end );
return make_iterator_range( new_begin, new_end );

Sequence copy_range( const ForwardRange& r );

Returns Sequence( begin(r), end(r) );

Class sub_range

The sub_range class inherits all its functionality from the iterator_range class. The sub_range class is often easier to use because one must specify the Forward Range template argument instead of an iterator. Moreover, the sub_range class can propagate constness since it knows what a corresponding const_iterator is.

Synopsis

namespace boost
{
    template< class ForwardRange >
    class sub_range : public iterator_range< typename range_result_iterator<ForwardRange>::type >
    {
    public: 
        typedef typename range_result_iterator<ForwardRange>::type iterator;
        typedef typename range_const_iterator<ForwardRange>::type  const_iterator;
    
    public: // construction, assignment
        template< class ForwardTraversalIterator >
        sub_range( ForwardTraversalIterator Begin, ForwardTraversalIterator End );

        template< class ForwardRange2 >
        sub_range( ForwardRange2& r );
         
        template< class ForwardRange2 >
        sub_range( const Range2& r );
         
        template< class ForwardRange2 >
        sub_range& operator=( ForwardRange2& r );

        template< class ForwardRange2 >
        sub_range& operator=( const ForwardRange2& r );    
    
    public:  // Forward Range functions 
        iterator        begin();
        const_iterator  begin() const;
        iterator        end();
        const_iterator  end() const;    
        
    public: // convenience 
        value_type&       front();
        const value_type& front() const;
        value_type&       back();
        const value_type& back() const;
        // for Random Access Range only: 
        value_type&       operator[]( size_type at );
        const value_type& operator[]( size_type at ) const;
    
    public:
        // rest of interface inherited from iterator_range
    };
    
} // namespace 'boost'

The class should be trivial to use as seen below. Imagine that we have an algorithm that searches for a sub-string in a string. The result is an iterator_range, that delimits the match. We need to store the result from this algorithm. Here is an example of how we can do it with and without sub_range 

    std::string str("hello");
    iterator_range<std::string::iterator> ir = find_first( str, "ll" );
    sub_range<std::string>               sub = find_first( str, "ll" );

(C) Copyright Thorsten Ottosen 2003-2004