XML/XPath: Difference between revisions
→{{header|C++}}: Replaced broken example with one that actually uses XPath.
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(→{{header|C++}}: Replaced broken example with one that actually uses XPath.) |
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=={{header|C++}}==
The following code uses each of the three tasks given to demonstrate a different method of handling the resulting node set from an XPath query.
{{libheader|LibXML}}
<lang cpp>#include <cassert>
#include <cstdlib>
#include <iostream>
#include <
#include <
#include <
#include <libxml/parser.h>
#include <libxml/tree.h>
#include <libxml/xmlerror.h>
#include <libxml/xmlstring.h>
#include <libxml/xmlversion.h>
#include <libxml/xpath.h>
#ifndef LIBXML_XPATH_ENABLED
# error libxml was not configured with XPath support
#endif
// Because libxml2 is a C library, we need a couple things to make it work
// well with modern C++:
// 1) a ScopeGuard-like type to handle cleanup functions; and
// 2) an exception type that transforms the library's errors.
// ScopeGuard-like type to handle C library cleanup functions.
template <typename F>
class [[nodiscard]] scope_exit
{
public:
// C++20: Constructor can (and should) be [[nodiscard]].
/*[[nodiscard]]*/ constexpr explicit scope_exit(F&& f) :
f_{std::move(f)}
{}
~scope_exit()
{
f_();
}
// Non-copyable, non-movable.
scope_exit(scope_exit const&) = delete;
scope_exit(scope_exit&&) = delete;
auto operator=(scope_exit const&) -> scope_exit& = delete;
auto operator=(scope_exit&&) -> scope_exit& = delete;
private:
F f_;
};
// Exception that gets last libxml2 error.
class libxml_error : public std::runtime_error
{
public:
libxml_error() : libxml_error(std::string{}) {}
explicit libxml_error(std::string message) :
std::runtime_error{make_message_(std::move(message))}
{}
private:
static auto make_message_(std::string message) -> std::string
{
if (auto const last_error = ::xmlGetLastError(); last_error)
{
if (not message.empty())
message += ": ";
message += last_error->message;
}
return message;
}
};
auto main() -> int
{
try
{
// Initialize libxml.
::xmlInitParser();
LIBXML_TEST_VERSION
auto const libxml_cleanup = scope_exit{[] { ::xmlCleanupParser(); }};
// Load and parse XML document.
auto const doc = ::xmlParseFile("test.xml");
if (not doc)
throw libxml_error{"failed to load document"};
auto const doc_cleanup = scope_exit{[doc] { ::xmlFreeDoc(doc); }};
// Create XPath context for document.
auto const xpath_context = ::xmlXPathNewContext(doc);
if (not xpath_context)
throw libxml_error{"failed to create XPath context"};
auto const xpath_context_cleanup = scope_exit{[xpath_context]
{ ::xmlXPathFreeContext(xpath_context); }};
// Task 1 ============================================================
{
std::cout << "Task 1:\n";
auto const xpath =
reinterpret_cast<::xmlChar const*>(u8"//item[1]");
// Create XPath object (same for every task).
auto xpath_obj = ::xmlXPathEvalExpression(xpath, xpath_context);
if (not xpath_obj)
throw libxml_error{"failed to evaluate XPath"};
auto const xpath_obj_cleanup = scope_exit{[xpath_obj]
{ ::xmlXPathFreeObject(xpath_obj); }};
// 'result' a xmlNode* to the desired node, or nullptr if it
// doesn't exist. If not nullptr, the node is owned by 'doc'.
auto const result = xmlXPathNodeSetItem(xpath_obj->nodesetval, 0);
if (result)
std::cout << '\t' << "node found" << '\n';
else
std::cout << '\t' << "node not found" << '\n';
}
// Task 2 ============================================================
{
std::cout << "Task 2:\n";
auto const xpath =
reinterpret_cast<::xmlChar const*>(u8"//price/text()");
// Create XPath object (same for every task).
auto xpath_obj = ::xmlXPathEvalExpression(xpath, xpath_context);
if (not xpath_obj)
throw libxml_error{"failed to evaluate XPath"};
auto const xpath_obj_cleanup = scope_exit{[xpath_obj]
{ ::xmlXPathFreeObject(xpath_obj); }};
// Printing the results.
auto const count =
xmlXPathNodeSetGetLength(xpath_obj->nodesetval);
for (auto i = decltype(count){0}; i < count; ++i)
{
auto const node =
xmlXPathNodeSetItem(xpath_obj->nodesetval, i);
assert(node);
auto const content = XML_GET_CONTENT(node);
assert(content);
// Note that reinterpret_cast here is a Bad Idea, because
// 'content' is UTF-8 encoded, which may or may not be the
// encoding cout expects. A *PROPER* solution would translate
// content to the correct encoding (or at least verify that
// UTF-8 *is* the correct encoding).
//
// But this "works" well enough for illustration.
std::cout << "\n\t" << reinterpret_cast<char const*>(content);
}
std::cout << '\n';
}
// Task 3 ============================================================
{
std::cout << "Task 3:\n";
auto const xpath =
reinterpret_cast<::xmlChar const*>(u8"//name");
// Create XPath object (same for every task).
auto xpath_obj = ::xmlXPathEvalExpression(xpath, xpath_context);
if (not xpath_obj)
throw libxml_error{"failed to evaluate XPath"};
auto const xpath_obj_cleanup = scope_exit{[xpath_obj]
{ ::xmlXPathFreeObject(xpath_obj); }};
// 'results' is a vector of pointers to the result nodes. The
// nodes pointed to are owned by 'doc'.
auto const results = [ns=xpath_obj->nodesetval]()
{
auto v = std::vector<::xmlNode*>{};
if (ns && ns->nodeTab)
v.assign(ns->nodeTab, ns->nodeTab + ns->nodeNr);
return v;
}();
std::cout << '\t' << "set of " << results.size()
<< " node(s) found" << '\n';
}
}
catch (std::exception const& x)
{
std::cerr << "ERROR: " << x.what() << '\n';
return EXIT_FAILURE;
}
}
</lang>
=={{header|Caché ObjectScript}}==
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