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# pixel_2d_locator_base Class Template Reference
[Models, Models]

`#include <locator.hpp>`

Inheritance diagram for pixel_2d_locator_base:

List of all members.

## Detailed Description

### template<typename Loc, typename XIterator, typename YIterator>
class boost::gil::pixel_2d_locator_base< Loc, XIterator, YIterator >

base class for models of PixelLocatorConcept

Pixel locator is similar to a pixel iterator, but allows for 2D navigation of pixels within an image view. It has a 2D difference_type and supports random access operations like:

         difference_type offset2(2,3);
         locator+=offset2;
         locator[offset2]=my_pixel;

In addition, each coordinate acts as a random-access iterator that can be modified separately: "++locator.x()" or "locator.y()+=10" thereby moving the locator horizontally or vertically.

It is called a locator because it doesn't implement the complete interface of a random access iterator. For example, increment and decrement operations don't make sense (no way to specify dimension). Also 2D difference between two locators cannot be computed without knowledge of the X position within the image.

This base class provides most of the methods and typedefs needed to create a model of a locator. GIL provides two locator models as subclasses of `pixel_2d_locator_base`. A memory-based locator, `memory_based_2d_locator` and a virtual locator, `virtual_2d_locator`. The minimum functionality a subclass must provide is this:

     class my_locator : public pixel_2d_locator_base<my_locator, ..., ...> {  // supply the types for x-iterator and y-iterator
            typedef ... const_t;                      // read-only locator

            template <typename Deref> struct add_deref {
                typedef ... type;                     // locator that invokes the Deref dereference object upon pixel access
                static type make(const my_locator& loc, const Deref& d);
            };

            my_locator();
            my_locator(const my_locator& pl);

            // constructors with dynamic step in y (and x). Only valid for locators with dynamic steps
            my_locator(const my_locator& loc, coord_t y_step);
            my_locator(const my_locator& loc, coord_t x_step, coord_t y_step, bool transpose);

            bool              operator==(const my_locator& p) const;

            // return _references_ to horizontal/vertical iterators. Advancing them moves this locator
            x_iterator&       x();
            y_iterator&       y();
            x_iterator const& x() const;
            y_iterator const& y() const;

            // return the vertical distance to another locator. Some models need the horizontal distance to compute it
            y_coord_t         y_distance_to(const my_locator& loc2, x_coord_t xDiff) const;

            // return true iff incrementing an x-iterator located at the last column will position it at the first
            // column of the next row. Some models need the image width to determine that.
            bool              is_1d_traversable(x_coord_t width) const;
     };

Models may choose to override some of the functions in the base class with more efficient versions.

## Public Types

typedef XIterator | x_iterator
typedef YIterator | y_iterator
typedef std::iterator_traits < x_iterator >::value_type | value_type
typedef std::iterator_traits < x_iterator >::reference | reference
typedef std::iterator_traits < x_iterator > ::difference_type | coord_t
typedef point2< coord_t > | difference_type
typedef difference_type | point_t
typedef point_t::template axis < 0 >::coord_t | x_coord_t
typedef point_t::template axis < 1 >::coord_t | y_coord_t
typedef difference_type | cached_location_t

## Public Member Functions

bool | operator!= (const Loc &p) const
x_iterator | x_at (x_coord_t dx, y_coord_t dy) const
x_iterator | x_at (const difference_type &d) const
y_iterator | y_at (x_coord_t dx, y_coord_t dy) const
y_iterator | y_at (const difference_type &d) const
Loc | xy_at (x_coord_t dx, y_coord_t dy) const
Loc | xy_at (const difference_type &d) const
template<std::size_t D>
axis< D >::iterator & | axis_iterator ()
template<std::size_t D>
axis< D >::iterator const & | axis_iterator () const
template<std::size_t D>
axis< D >::iterator | axis_iterator (const point_t &p) const
reference | operator() (x_coord_t dx, y_coord_t dy) const
reference | operator[] (const difference_type &d) const
reference | operator* () const
Loc & | operator+= (const difference_type &d)
Loc & | operator-= (const difference_type &d)
Loc | operator+ (const difference_type &d) const
Loc | operator- (const difference_type &d) const
cached_location_t | cache_location (const difference_type &d) const
cached_location_t | cache_location (x_coord_t dx, y_coord_t dy) const

## Static Public Attributes

static const std::size_t | num_dimensions = 2

## Friends

class | pixel_2d_locator

The documentation for this class was generated from the following file:

- locator.hpp