Readable Property Map

A Readable Property Map provides read-access to the value object associated with a given key via a call to the get() function. The get() function returns a copy of the value object.

Refinement of

Copy Constructible

Notation

PMap	A type that is a model of Readable Property Map.
pmap	An object of type PMap.
key	An object of type boost::property_traits<PMap>::key_type.

Associated Types

Value Type	boost::property_traits<PMap>::value_type	The type of the property.
Reference Type	boost::property_traits<PMap>::reference	A type that is convertible to the value type.
Key Type	boost::property_traits<PMap>::key_type	The type of the key object used to look up the property. The property map may be templated on the key type, in which case this typedef can be void.
Property Map Category	boost::property_traits<PMap>::category	The category of the property: a type convertible to readable_property_map_tag.

Valid Expressions

Name	Expression	Return Type	Description
Get Property Value	get(pmap, key)	reference	Lookup the property of the object identified by key.

Concept Checking Class

  template <class PMap, class Key>
  struct ReadablePropertyMapConcept
  {
    typedef typename property_traits<PMap>::key_type key_type;
    typedef typename property_traits<PMap>::category Category;
    typedef boost::readable_property_map_tag ReadableTag;
    void constraints() {
      function_requires< ConvertibleConcept<Category, ReadableTag> >();

      val = get(pmap, k);
    }
    PMap pmap;
    Key k;
    typename property_traits<PMap>::value_type val;
  };

Design Notes

At various times the name "read-only" was considered for this concept. However, that name is inappropriate because concepts are inherently positive, not negative. This becomes obvious when the user defines the Read Write Property Map, which refines both the Readable Property Map and the Writable Property Map concept. It would not make much sense to combine "read-only" and "write-only" concepts!