C++解析xml的開源庫有很多，在此我就不一一列舉了，今天主要說下Rapidxml，我使用這個庫也并不是很多，如有錯誤之處還望大家能夠之處，謝謝。
附：
官方鏈接：http://rapidxml.sourceforge.net/
官方手冊：http://rapidxml.sourceforge.net/manual.html
之前有一次用到，碰到了個"坑"，當時時間緊迫并未及時查找，今天再次用到這個庫，對這樣的"坑"不能踩第二次，因此我決定探個究竟。
先寫兩段示例：
創建xm：
void CreateXml(){ rapidxml::xml_document<> doc; auto nodeDecl = doc.allocate_node(rapidxml::node_declaration); nodeDecl->append_attribute(doc.allocate_attribute("version", "1.0")); nodeDecl->append_attribute(doc.allocate_attribute("encoding", "UTF-8")); doc.append_node(nodeDecl);//添加xml聲明 auto nodeRoot = doc.allocate_node(rapidxml::node_element, "Root");//創建一個Root節點 nodeRoot->append_node(doc.allocate_node(rapidxml::node_comment, NULL, "編程語言"));//添加一個注釋內容到Root，注釋沒有name 所以第二個參數為NULL auto nodeLangrage = doc.allocate_node(rapidxml::node_element, "language", "This is C language");//創建一個language節點 nodeLangrage->append_attribute(doc.allocate_attribute("name", "C"));//添加一個name屬性到language nodeRoot->append_node(nodeLangrage); //添加一個language到Root節點 nodeLangrage = doc.allocate_node(rapidxml::node_element, "language", "This is C++ language");//創建一個language節點 nodeLangrage->append_attribute(doc.allocate_attribute("name", "C++"));//添加一個name屬性到language nodeRoot->append_node(nodeLangrage); //添加一個language到Root節點 doc.append_node(nodeRoot);//添加Root節點到Document std::string buffer; rapidxml::print(std::back_inserter(buffer), doc, 0); std::ofstream outFile("language.xml"); outFile << buffer; outFile.close();}結果：
<?xml version="1.0" encoding="UTF-8"?> <Root> <!--編程語言--> <language name="C">This is C language</language> <language name="C++">This is C++ language</language> </Root>
修改xml：
void MotifyXml(){ rapidxml::file<> requestFile("language.xml");//從文件加載xml rapidxml::xml_document<> doc; doc.parse<0>(requestFile.data());//解析xml auto nodeRoot = doc.first_node();//獲取第一個節點，也就是Root節點 auto nodeLanguage = nodeRoot->first_node("language");//獲取Root下第一個language節點 nodeLanguage->first_attribute("name")->value("Motify C");//修改language節點的name屬性為 Motify C std::string buffer; rapidxml::print(std::back_inserter(buffer), doc, 0); std::ofstream outFile("MotifyLanguage.xml"); outFile << buffer; outFile.close();}結果：
<Root> <language name="Motify C">This is C language</language> <language name="C++">This is C++ language</language> </Root>
由第二個結果得出：
第一個language的name屬性確實改成我們所期望的值了，不過不難發現xml的聲明和注釋都消失了。是怎么回事呢？這個問題也困擾了我一段時間，既然是開源庫，那我們跟一下看看他都干了什么，從代碼可以看出可疑的地方主要有兩處：print和parse，這兩個函數均需要提供一個flag，這個flag到底都干了什么呢，從官方給的教程來看 均使用的0，既然最終執行的是print我們就從print開始調試跟蹤吧
找到了找到print調用的地方：
template<class OutIt, class Ch> inline OutIt print(OutIt out, const xml_node<Ch> &node, int flags = 0) { return internal::print_node(out, &node, flags, 0); }繼續跟蹤：
// Print node template<class OutIt, class Ch> inline OutIt print_node(OutIt out, const xml_node<Ch> *node, int flags, int indent) { // Print proper node type switch (node->type()) { // Document case node_document: out = print_children(out, node, flags, indent); break; // Element case node_element: out = print_element_node(out, node, flags, indent); break; // Data case node_data: out = print_data_node(out, node, flags, indent); break; // CDATA case node_cdata: out = print_cdata_node(out, node, flags, indent); break; // Declaration case node_declaration: out = print_declaration_node(out, node, flags, indent); break; // Comment case node_comment: out = print_comment_node(out, node, flags, indent); break; // Doctype case node_doctype: out = print_doctype_node(out, node, flags, indent); break; // Pi case node_pi: out = print_pi_node(out, node, flags, indent); break; // Unknown default: assert(0); break; } // If indenting not disabled, add line break after node if (!(flags & print_no_indenting)) *out = Ch('/n'), ++out; // Return modified iterator return out; }跟進print_children 發現這實際是個遞歸，我們繼續跟蹤
// Print element nodetemplate<class OutIt, class Ch>inline OutIt print_element_node(OutIt out, const xml_node<Ch> *node, int flags, int indent){ assert(node->type() == node_element); // Print element name and attributes, if any if (!(flags & print_no_indenting)) ...//省略部分代碼 return out;}我們發現第8行有一個&判斷 查看print_no_indenting的定義：
// Printing flagsconst int print_no_indenting = 0x1; //!< Printer flag instructing the printer to suppress indenting of XML. See print() function.
據此我們就可以分析了，按照開發風格統一的思想，parse也應該有相同的標志定義
省略分析parse流程..
我也順便去查看了官方文檔，確實和我預想的一樣，貼一下頭文件中對這些標志的描述，詳細信息可參考官方文檔
// Parsing flags //! Parse flag instructing the parser to not create data nodes. //! Text of first data node will still be placed in value of parent element, unless rapidxml::parse_no_element_values flag is also specified. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_no_data_nodes = 0x1; //! Parse flag instructing the parser to not use text of first data node as a value of parent element. //! Can be combined with other flags by use of | operator. //! Note that child data nodes of element node take precendence over its value when printing. //! That is, if element has one or more child data nodes <em>and</em> a value, the value will be ignored. //! Use rapidxml::parse_no_data_nodes flag to prevent creation of data nodes if you want to manipulate data using values of elements. //! <br><br> //! See xml_document::parse() function. const int parse_no_element_values = 0x2; //! Parse flag instructing the parser to not place zero terminators after strings in the source text. //! By default zero terminators are placed, modifying source text. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_no_string_terminators = 0x4; //! Parse flag instructing the parser to not translate entities in the source text. //! By default entities are translated, modifying source text. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_no_entity_translation = 0x8; //! Parse flag instructing the parser to disable UTF-8 handling and assume plain 8 bit characters. //! By default, UTF-8 handling is enabled. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_no_utf8 = 0x10; //! Parse flag instructing the parser to create XML declaration node. //! By default, declaration node is not created. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_declaration_node = 0x20; //! Parse flag instructing the parser to create comments nodes. //! By default, comment nodes are not created. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_comment_nodes = 0x40; //! Parse flag instructing the parser to create DOCTYPE node. //! By default, doctype node is not created. //! Although W3C specification allows at most one DOCTYPE node, RapidXml will silently accept documents with more than one. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_doctype_node = 0x80; //! Parse flag instructing the parser to create PI nodes. //! By default, PI nodes are not created. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_pi_nodes = 0x100; //! Parse flag instructing the parser to validate closing tag names. //! If not set, name inside closing tag is irrelevant to the parser. //! By default, closing tags are not validated. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_validate_closing_tags = 0x200; //! Parse flag instructing the parser to trim all leading and trailing whitespace of data nodes. //! By default, whitespace is not trimmed. //! This flag does not cause the parser to modify source text. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_trim_whitespace = 0x400; //! Parse flag instructing the parser to condense all whitespace runs of data nodes to a single space character. //! Trimming of leading and trailing whitespace of data is controlled by rapidxml::parse_trim_whitespace flag. //! By default, whitespace is not normalized. //! If this flag is specified, source text will be modified. //! Can be combined with other flags by use of | operator. //! <br><br> //! See xml_document::parse() function. const int parse_normalize_whitespace = 0x800; // Compound flags //! Parse flags which represent default behaviour of the parser. //! This is always equal to 0, so that all other flags can be simply ored together. //! Normally there is no need to inconveniently disable flags by anding with their negated (~) values. //! This also means that meaning of each flag is a <i>negation</i> of the default setting. //! For example, if flag name is rapidxml::parse_no_utf8, it means that utf-8 is <i>enabled</i> by default, //! and using the flag will disable it. //! <br><br> //! See xml_document::parse() function. const int parse_default = 0; //! A combination of parse flags that forbids any modifications of the source text. //! This also results in faster parsing. However, note that the following will occur: //! <ul> //! <li>names and values of nodes will not be zero terminated, you have to use xml_base::name_size() and xml_base::value_size() functions to determine where name and value ends</li> //! <li>entities will not be translated</li> //! <li>whitespace will not be normalized</li> //! </ul> //! See xml_document::parse() function. const int parse_non_destructive = parse_no_string_terminators | parse_no_entity_translation; //! A combination of parse flags resulting in fastest possible parsing, without sacrificing important data. //! <br><br> //! See xml_document::parse() function. const int parse_fastest = parse_non_destructive | parse_no_data_nodes; //! A combination of parse flags resulting in largest amount of data being extracted. //! This usually results in slowest parsing. //! <br><br> //! See xml_document::parse() function. const int parse_full = parse_declaration_node | parse_comment_nodes | parse_doctype_node | parse_pi_nodes | parse_validate_closing_tags;
根據以上提供的信息我們改下之前的源代碼：
將
doc.parse<0>(requestFile.data());//解析xml auto nodeRoot = doc.first_node("");//獲取第一個節點，也就是Root節點 改為
doc.parse<rapidxml::parse_declaration_node | rapidxml::parse_comment_nodes | rapidxml::parse_non_destructive>(requestFile.data());//解析xml auto nodeRoot = doc.first_node("Root");//獲取第一個節點，也就是Root節點這里解釋一下，parse加入了三個標志，分別是告訴解析器創建聲明節點、告訴解析器創建注釋節點、和不希望解析器修改傳進去的數據，第二句是當有xml的聲明時，默認的first_node并不是我們期望的Root節點，因此通過傳節點名來找到我們需要的節點。
注：
1、這個庫在append的時候并不去判斷添加項（節點、屬性等）是否存在
2、循環遍歷時對項（節點、屬性等）進行修改會導致迭代失效
總結：用別人寫的庫，總會有些意想不到的問題，至今我只遇到了這些問題，如果還有其它問題歡迎補充，順便解釋下"坑"并不一定是用的開源庫有問題，更多的時候可能是還沒有熟練的去使用這個工具。
感謝rapidxml的作者，為我們提供一個如此高效便利的工具。
以上這篇淺談使用Rapidxml 庫遇到的問題和分析過程(分享)就是小編分享給大家的全部內容了，希望能給大家一個參考，也希望大家多多支持武林網。
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