c++ - 第二个孩子的 XML 路径

Question

我想通过路径为 ptree 内的子项添加一个新值（无迭代）

例如：

#include <iostream>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/xml_parser.hpp>

int main() {
  using boost::property_tree::ptree;

  ptree main;
  ptree children;
  children.add("Foo", "bar");
  main.add_child("child", children);
  main.add_child("child", children);

  ptree newChildren("Foo");
  main.put("child{2}.Foo", newChildren.data()); // <-- Access second element?

  // Output
  boost::property_tree::xml_writer_settings<std::string> settings('\t', 1);
  boost::property_tree::write_xml(std::cout, main, settings);

  std::cin.ignore();
  return 0;
}

问题是，我无法通过路径访问第二个孩子。有什么格式有效吗？

我的意思是这 {2} 部分：

"child{2}.Foo"

我试过 <2>, [2], (2)...没有运气... :(

对我有希望吗？谢谢！

Answer 1

我再重复一遍：Boost 中没有 xml 解析器/库。

您正在（ab）使用的是Boost Property Tree。正如您所发现的，它是一个“属性树”库，这意味着它可以做一些事情。包括写入和读取属性树。

如果您想要通用 XML 的东西，请考虑使用 XML 库（我应该在 C++ 中使用什么 XML 解析器？）。

无需再费周折：

Live On Coliru

#include <iostream>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/xml_parser.hpp>

int main() {
    using boost::property_tree::ptree;

    ptree pt;

    {
        ptree children; children.add("Foo", "bar");
        pt.add_child("child", children);
        pt.add_child("child", children).put("MARK","so we know it's the second");
    }

    // Output
    boost::property_tree::xml_writer_settings<std::string> settings('\t', 1);
    //boost::property_tree::write_xml(std::cout, pt, settings);

    auto child1 = std::find_if(pt.begin(),        pt.end(), [](auto& node) { return node.first == "child"; });
    auto child2 = std::find_if(std::next(child1), pt.end(), [](auto& node) { return node.first == "child"; });

    if (child2 != pt.end())
    {
        boost::property_tree::write_xml(std::cout, child2->second, settings);

        ptree newChildren("Foo");
        child2->second.put("Sub.Foo", newChildren.data()).put("BYE", "ALL DONE"); // <-- Access second element?
        boost::property_tree::write_xml(std::cout << "\n\nAFTER EDITING:\n", child2->second, settings);
    }
}

印刷：

<?xml version="1.0" encoding="utf-8"?>
<Foo>bar</Foo>
<MARK>so we know it&apos;s the second</MARK>


AFTER EDITING:
<?xml version="1.0" encoding="utf-8"?>
<Foo>bar</Foo>
<MARK>so we know it&apos;s the second</MARK>
<Sub>
    <Foo>
        Foo
        <BYE>ALL DONE</BYE>
    </Foo>
</Sub>

更新

使用辅助函数改进样式：

Live On Coliru

#include <iostream>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/xml_parser.hpp>

template <typename Tree, typename Out>
Out find_all(Tree& pt, typename Tree::path_type path, Out out) {
    if (path.empty()) {
        *out++ = pt;
        return out;
    }

    auto head = path.reduce();
    for (auto& child : pt)
        if (child.first == head)
            out = find_all(child.second, path, out);

    return out;
}

int main() {
    using boost::property_tree::ptree;

    ptree pt;

    {
        ptree children; children.add("Foo", "bar");
        pt.add_child("child", children);
        pt.add_child("child", children).put("MARK","so we know it's the second");
    }

    // Output
    boost::property_tree::xml_writer_settings<std::string> settings('\t', 1);
    //boost::property_tree::write_xml(std::cout, pt, settings);

    std::vector<std::reference_wrapper<ptree> > matches;
    find_all(pt, "child", back_inserter(matches));

    ptree& child2 = matches.at(1);
    child2.put("BYE", "ALL DONE");

    boost::property_tree::write_xml(std::cout, child2, settings);
}

印刷

<?xml version="1.0" encoding="utf-8"?>
<Foo>bar</Foo>
<MARK>so we know it&apos;s the second</MARK>
<BYE>ALL DONE</BYE>