libs/range/doc/reference/adaptors/type_erased.qbk

[/
    Copyright 2010 Neil Groves
    Distributed under the Boost Software License, Version 1.0.
    (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
/]
[section:type_erased type_erased]

[table
    [[Syntax] [Code]]
    [[Pipe] [`rng | boost::adaptors::type_erased<Value, Traversal, Reference, Difference, Buffer>()`]]
    [[Function] [`boost::adaptors::type_erase(rng, boost::adaptors::type_erased<Value, Traversal, Reference, Difference, Buffer>)`]]
]

Please note that it is frequently unnecessary to use the `type_erased` adaptor. It is often better to use the implicit conversion to `any_range`.

Let `Rng` be the type of `rng`.

* [*Template parameters:]
    * `Value` is the `value_type` for the `any_range`. If this is set to boost::use_default, `Value` will be calculated from the
        range type when the adaptor is applied.
    * `Traversal` is the tag used to identify the traversal of the resultant range. Frequently it is desireable to set a traversal category lower than the source container or range to maximize the number of ranges that can convert to the `any_range`. If this is left as boost::use_default then `Traversal` will be `typename boost::iterator_traversal<boost::range_iterator<Rng>::type>::type`
    * `Reference` is the `reference` for the `any_range`. `boost::use_default` will equate to `typename range_reference<Rng>::type`.
    * `Difference` is the `difference_type` for the any_range. `boost::use_default` will equate to `typename boost::range_difference<Rng>::type`
    * `Buffer` is the storage used to allocate the underlying iterator wrappers. This can typically be ignored, but is available as a template parameter for customization. Buffer must be a model of the `AnyIteratorBufferConcept`.
* [*Precondition:]  `Traversal` is one of `{ boost::use_default, boost::single_pass_traversal_tag, boost::forward_traversal_tag, boost::bidirectional_traversal_tag, boost::random_access_traversal_tag }`
* [*Returns:] The returned value is the same as `typename any_range_type_generator< Rng, Value, Traversal, Reference, Difference, Buffer >` that represents `rng` in a type-erased manner.
* [*Range Category:] __single_pass_range__
* [*Returned Range Category:] if `Traversal` was specified as `boost::use_default` then `typename boost::iterator_traversal<boost::range_iterator<Rng>::type>::type`, otherwise `Traversal`.

[heading AnyIteratorBufferConcept]
``
class AnyIteratorBufferConcept
{
public:
    AnyIteratorBufferConcept();
    ~AnyIteratorBufferConcept();

    // bytes is the requested size to allocate. This function
    // must return a pointer to an adequate area of memory.
    // throws: bad_alloc
    //
    // The buffer will only ever have zero or one
    // outstanding memory allocations.
    void* allocate(std::size_t bytes);

    // deallocate this buffer
    void deallocate();
};
``

[section:type_erased_example type-erased example]
``
#include <boost/range/adaptor/type_erased.hpp>
#include <boost/range/algorithm/copy.hpp>
#include <boost/assign.hpp>
#include <boost/foreach.hpp>
#include <algorithm>
#include <iostream>
#include <list>
#include <vector>

// The client interface from an OO perspective merely requires a sequence
// of integers that can be forward traversed
typedef boost::any_range<
    int
  , boost::forward_traversal_tag
  , int
  , std::ptrdiff_t
> integer_range;

namespace server
{
    void display_integers(const integer_range& rng)
    {
        boost::copy(rng,
                    std::ostream_iterator<int>(std::cout, ","));

        std::cout << std::endl;
    }
}

namespace client
{
    void run()
    {
        using namespace boost::assign;
        using namespace boost::adaptors;

        // Under most conditions one would simply use an appropriate
        // any_range as a function parameter. The type_erased adaptor
        // is often superfluous. However because the type_erased
        // adaptor is applied to a range, we can use default template
        // arguments that are generated in conjunction with the
        // range type to which we are applying the adaptor.

        std::vector<int> input;
        input += 1,2,3,4,5;

        // Note that this call is to a non-template function
        server::display_integers(input);

        std::list<int> input2;
        input2 += 6,7,8,9,10;

        // Note that this call is to the same non-tempate function
        server::display_integers(input2);

        input2.clear();
        input2 += 11,12,13,14,15;

        // Calling using the adaptor looks like this:
        // Notice that here I have a type_erased that would be a
        // bidirectional_traversal_tag, but this is convertible
        // to the forward_traversal_tag equivalent hence this
        // works.
        server::display_integers(input2 | type_erased<>());

        // However we may simply wish to define an adaptor that
        // takes a range and makes it into an appropriate
        // forward_traversal any_range...
        typedef boost::adaptors::type_erased<
            boost::use_default
          , boost::forward_traversal_tag
        > type_erased_forward;

        // This adaptor can turn other containers with different
        // value_types and reference_types into the appropriate
        // any_range.

        server::display_integers(input2 | type_erased_forward());
    }
}

int main(int argc, const char* argv[])
{
    client::run();
    return 0;
}
``
[endsect]

This would produce the output:
``
1,2,3,4,5
6,7,8,9,10
11,12,13,14,15
11,12,13,14,15
``
[endsect]