Document Traversal

document_traversal Class

document_traversal template class is a facade that provides access to most features of SVG++ library.

document_traversal contains only static methods. Each document_traversal method receives context and XML element parameters. Context is a user defined object that will receive parsed data. XML element has type of XML element in chosen XML parser.

Named template parameters of document_traversal class allows to configure almost each aspect of SVG processing.

template<class Args...>
struct document_traversal
  template<class XMLElement, class Context>
  static bool load_document(XMLElement const & xml_root_element, Context & context)
    { return load_expected_element(xml_root_element, context, tag::element::svg()); }

  template<class XMLElement, class Context, class ElementTag>
  static bool load_expected_element(
    XMLElement const & xml_element,
    Context & context,
    ElementTag expected_element);

  template<class RefArgs...>
  struct load_referenced_element
    template<class XMLElement, class Context>
    static bool load(XMLElement const & xml_element, Context & parent_context);


template<class XMLElement, class Context>
static bool load_document(XMLElement const & xml_root_element, Context & context);
load_document - is a shortcut for load_expected_element that receives root element (svg) of SVG document as xml_root_element parameter.
template<class XMLElement, class Context, class ElementTag>
static bool load_expected_element(
  XMLElement const & xml_element,
  Context & context,
  ElementTag expected_element);

load_expected_element loads xml_element and its descendands and passes parsed data to context.

load_expected_element expects that XML name of xml_element corresponds to ElementTag. Otherwise exception unexpected_element_error is thrown (see Error Policy).

template<class RefArgs...>
template<class XMLElement, class Context>
static bool load_referenced_element<RefArgs...>::load(
  XMLElement const & xml_element,
  Context & parent_context);

load_referenced_element mainly used to load SVG elements that are referenced by other SVG element, e.g. use element or gradients. Unlike load_expected_element, allowed XML name of xml_element isn’t limited to one and is passed as expected_elements sequence.

Named class template parameters of document_traversal::load_referenced_element

expected_elements (required)

Value is Associative Sequence. Contains list of tags of expected elements for xml_element. If XML name of xml_element doesn’t correspond to any from the list, exception unexpected_element_error (see Error Policy) is thrown.


traits::reusable_elements contains list of elements, that can be referenced by use element.

referencing_element (optional)
Value is tag of element that references xml_element. It is used only if Viewport Policy requires processing of viewport by SVG++ (for correct processing of svg and symbol elements, referenced by image or use elements).
processed_elements or ignored_elements (optional)
Only one may be set. See description of document_traversal parameters with the same names. Allows to override document_traversal settings processed_elements/ignored_elements for the passed SVG element. Child elements will be processed with document_traversal settings.

document_traversal Named Class Template Parameters

ignored_elements and processed_elements

One of them must be set to limit SVG elements processed. It must be model of Associative Sequence (e.g. boost::mpl::set), containing element tags.

If processed_elements is set, then only the listed elements are processed, otherwise if ignored_elements is set, then only non-listed elements are processed.

ignored_attributes and processed_attributes

One of them must be set to limit SVG attributes processed. It must be model of Associative Sequence, containing attribute tags. Also it may contain pairs <element tag, attribute tag> like this boost::mpl::pair<tag::element::g, tag::attribute::transform>, such pair is matched if both processed element and processed attribute matches tags.

If processed_attributes is set, then only listed attributes are processed, otherwise if ignored_attributes is set, then only non-listed attributes are processed.

passthrough_attributes (optional)
Is a Associative Sequence, that contains attribute tags. Values of listed attributes aren’t parsed by SVG++, and passed to the user code as string.
context_factories (optional)
See Context Factories.
attribute_traversal_policy (optional)
See Attribute Traversal Policy.

Context Factories

When document_traversal traverses SVG tree, it, for each SVG element met, chooses context to be used to pass attribute values and character data by corresponding event policy. To configure this behavior context_factories parameter is used.

context_factories parameter accepts Metafunction Class, that receives:

  • ParentContext - parent context type (context used for parent SVG element);
  • ElementTag - element tag (corresponds to the SVG element met),

and returns Context Factory.

  typename context_factories::template apply<ParentContext, ElementTag>::type

SVG++ provides several Context Factories:

template<class ParentContext, class ElementTag>
class factory::context::same;
The new context object isn’t created, parent context will be used. on_enter_element(ElementTag()) and on_exit_element() methods of parent context will be called at the start and at the end of element processing respectively.
template<class ChildContext>
class factory::context::on_stack;

The new object of type ChildContext is created on stack. Constructor of the object is passed with the reference to the parent context. After element processing is finished, method ChildContext::on_exit_element() is called before destruction. Lifetime of context object matches processing of SVG element content (element attributes, child elements and text nodes).

factory::context::on_stack_with_xml_element is the same as factory::context::on_stack, but ChildContext constructor receives second parameter - XML element. Its type depends on XML parser used.

  class ElementTag,
  class ChildContextPtr,
  class ChildContext = typename boost::pointee<ChildContextPtr>::type
class get_ptr_from_parent;
Pointer ChildContextPtr to context object is requested from parent context by call to the method get_child_context(ElementTag()). Pointer can be raw pointer or some smart pointer. ChildContext::on_exit_element() is called after element processing is finished.

Attribute Traversal Policy

Attribute Traversal Policy configures order and other aspects of SVG attributes and CSS properties processing.

struct attribute_traversal_policy_concept
  static const bool parse_style = /* true or false */;
  static const bool css_hides_presentation_attribute = /* true or false */;

  typedef /* Metafunction class */ get_priority_attributes_by_element;
  typedef /* Metafunction class */ get_deferred_attributes_by_element;
  typedef /* Metafunction class */ get_required_attributes_by_element;
parse_style = true
Contents of style attribute is parsed as a sequence of semicolon-separated pairs property-value.
css_hides_presentation_attribute = true

Is checked only if parse_style = true.

If the same property is set both in style attribute and as a presentation attribute, then only value in style attribute will be parsed (it has higher priority according to SVG).

If css_hides_presentation_attribute = false, then memory usage is lower, but both values (from style attribute and a presentation attribute) of the same property will be parsed and passed to the user code in the arbitrary order.

get_priority_attributes_by_element, get_deferred_attributes_by_element and get_required_attributes_by_element

They are Metafunction classes, receiving element tag and returning Forward Sequence of attribute tags.

Attributes, returned by get_priority_attributes_by_element metafunction for current element, will be processed before all others in the same order as in sequence.

Attributes, returned by get_deferred_attributes_by_element metafunction for current element, will be processed after all others in the same order as in sequence.


Sequences, returned by get_priority_attributes_by_element and get_deferred_attributes_by_element, besides attribute tags may contain elements like notify_context<EventTag>. EventTag is an arbitrary tag, that will be passed to notify method of the context. notify(EventTag()) will be called right after all previous attributes in the sequence are processed.

If element lacks any attribute from the sequence, returned by get_required_attributes_by_element metafunction for this element, then required_attribute_not_found_error exception will be thrown (see Error Policy). SVG++ contains definition of traits::element_required_attributes metafunction that returns mandatory attributes for element, according to SVG Specification. It can be used like this:

struct my_attribute_traversal_policy
  /* ... other members ... */
  typedef boost::mpl::quote1<traits::element_required_attributes>