| # 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:

- Class `iterator_range`
- Class `sub_range`

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;

        };

        // 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 );

        // convenience
        template< class Sequence, class ForwardRange >
        Sequence copy_range( const ForwardRange& r );

        template< class Sequence, class ForwardRange, class Func >
        Sequence transform_range( const ForwardRange& r, Func func );

    } // 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) );`

`Sequence copy_range( const ForwardRange& r );`

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

`Sequence transform_range( const ForwardRange& r, Func func );`

> Effects
> `Sequence seq;`
> `std::transform( begin(r), end(r), std::back_inserter(seq), func );`
> `return seq;`

# 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:
            iterator        begin();
            const_iterator  begin() const;
            iterator        end();
            const_iterator  end() 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