如何循環在提升XML結構:: property_tree

Question

我的線沿線的XML結構：這是被讀入一個boost::property_tree

<root> 
<SomeElement> 
    <AnotherElement> 
    <ElementIWant x="1" y="1"/> 
    </AnotherElement> 
</SomeElement> 
<SomeElement> 
    <AnotherElement> 
    <ElementIWant x="1" y="1"/> 
    <ElementIWant x="2" y="1"/> 
    <ElementIWant x="3" y="1"/> 
    </AnotherElement> 
</SomeElement> 
</root> 

，有1..Many<SomeElement> S，然後在該元件內的任意深度有可能是1..Many<ElementIWant>小號

有一種方法，它們的APPEA順序在<ElementIWant>直接進行迭代（在一個單一的循環） r在文檔中？

我已經看過equal_range

void iterateOverPoints() 
{ 
    const char* test = 
    "<?xml version=\"1.0\" encoding=\"utf-8\"?><root>" 
     "<SomeElement>" 
     "<AnotherElement>" 
     "<ElementIWant x=\"1\" y=\"1\"/>" 
     "</AnotherElement>" 
     "</SomeElement>" 
     "<SomeElement>" 
     "<AnotherElement>" 
     "<ElementIWant x=\"1\" y=\"1\"/>" 
     "<ElementIWant x=\"2\" y=\"1\"/>" 
     "<ElementIWant x=\"3\" y=\"1\"/>" 
     "</AnotherElement>" 
     "</SomeElement>" 
    "</root>"; 

    boost::property_tree::ptree message; 
    std::istringstream toParse(test); 
    boost::property_tree::read_xml(toParse,result_tree); 

    //Now we need to locate the point elements and set the x/y accordingly. 
    std::pair< boost::property_tree::ptree::const_assoc_iterator, 
       boost::property_tree::ptree::const_assoc_iterator > result = 
     message.equal_range("ElementIWant"); 

    for(boost::property_tree::ptree::const_assoc_iterator it = result.first; 
      it != result.second; ++it) 
    { 
     std::cout << it->first << " : "; 
     const boost::property_tree::ptree& x = it->second.get_child("<xmlattr>.x"); 
     const boost::property_tree::ptree& y = it->second.get_child("<xmlattr>.y"); 
     std::cout << x.get_value<int>() << "," << y.get_value<int>() << "\n"; 
    } 

    return; 
} 

但是它似乎無法返回節點（我懷疑是因爲equal_range工作在提供的樹節點的水平），這使我對上面的問題。 ...

Answer 1

不可能直接遍歷所有元素; the documentation says

無法遍歷整個樹。

現在，您可以使用遞歸，並在每個級別應用STL算法來模擬該算法;它不適合你在我下面的樣品中單迴路這樣的要求，但它的工作原理：

template <typename InputIt, typename OutputIt, typename Compare> 
void collect(InputIt first, InputIt last, OutputIt dest, Compare comp) 
{ 
    typedef typename std::iterator_traits<InputIt>::reference reference; 

    std::copy_if (
     first, last, dest, 
     [comp] (reference what) { return comp(what.first); }); 

    std::for_each (
     first, last, 
     [dest, comp] (reference what) { collect(what.second.begin(), what.second.end(), dest, comp); }); 
} 


std::vector<std::pair<std::string, ptree>> match; 

collect(
    xml.begin(), xml.end(), std::back_inserter(match), 
    [] (const std::string& key) { return key == "ElementIWant"; }); 

for (auto pair: match) 
{ 
    std::cout << pair.first << std::endl; 
} 

---

這裏是一個版本，是「完全」遞歸和保護出現的順序：

template <typename InputIt, typename OutputIt, typename Compare> 
void collect_recursive(InputIt first, InputIt last, OutputIt dest, Compare comp) 
{ 
    typedef typename std::iterator_traits<InputIt>::reference reference; 

    if (first == last) 
    { 
     return; 
    } 

    auto begin = first->second.begin(); 
    auto end = first->second.end(); 

    if (begin != end) 
    { 
     collect_recursive (begin, end, dest, comp); 
    } 

    if (comp (first->first)) 
    { 
     dest = *first; 
    } 

    collect_recursive (++first, last, dest, comp); 
}