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.. _error-handling:
Error handling
=================
Returned values
-----------------------
In most cases when SVG++ methods returns ``bool`` values, they can be used as an alternative to exceptions
- if method returns ``false``, then calling method immediately returns with ``false`` result and so on.
Error reporting is controlled by *Error Policy*. ``policy::error::raise_exception`` used by default
throws exception objects, derived from ``std::exception`` and ``boost::exception``.
In this case only ``true`` may be returned as a result code.
Default Error Handling
----------------------------
Default ``policy::error::raise_exception`` uses Boost.Exception for transporting arbitrary
data to the catch site.
``boost::error_info`` uses tags ``tag::error_info::xml_element`` and ``tag::error_info::xml_attribute``
to pass information about place in SVG document where error occured alongside with the exception object.
Value type depends on XML parser and *XML Policy* used.
Example of SVG++ exception handling when RapidXML NS parser is used::
typedef rapidxml_ns::xml_node<> const * XMLElement;
try
{
/* ... */
document_traversal</* ... */>::load_document(/* ... */);
}
catch(svgpp::exception_base const & e)
{
typedef boost::error_info<svgpp::tag::error_info::xml_element, XMLElement> element_error_info;
std::cerr << "Error reading SVG";
if (XMLElement const * element = boost::get_error_info<element_error_info>(e))
std::cerr
<< " in element '"
<< std::string((*element)->name(), (*element)->name() + (*element)->name_size())
<< "'";
std::cerr << ": " << e.what() << "\n";
}
.. _error_policy:
Error Policy Concept
---------------------------
::
struct error_policy
{
typedef /* ... */ context_type;
template<class XMLElement, class ElementName>
static bool unknown_element(
context_type const &,
XMLElement const & element,
ElementName const & name);
template<class XMLAttributesIterator, class AttributeName>
static bool unknown_attribute(context_type &,
XMLAttributesIterator const & attribute,
AttributeName const & name,
BOOST_SCOPED_ENUM(detail::namespace_id) namespace_id,
tag::source::attribute);
template<class XMLAttributesIterator, class AttributeName>
static bool unknown_attribute(context_type &,
XMLAttributesIterator const & attribute,
AttributeName const & name,
tag::source::css);
static bool unexpected_attribute(context_type &,
detail::attribute_id id, tag::source::attribute);
template<class AttributeTag>
static bool required_attribute_not_found(context_type &, AttributeTag);
template<class AttributeTag, class AttributeValue>
static bool parse_failed(context_type &, AttributeTag,
AttributeValue const & value);
template<class XMLElement>
static bool unexpected_element(context_type &, XMLElement const & element);
template<class AttributeTag>
static bool negative_value(context_type &, AttributeTag);
typedef /* ... */ intercepted_exception_type;
template<class XMLElement>
static bool add_element_info(intercepted_exception_type & e,
XMLElement const & element);
};
If *Error Policy* method returns ``true``, then SVG++ continues SVG processing skipping the part with the error.
In some cases it may lead to problems in further processing.
If method returns ``false``, then processing immediately stops.
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