C++解析xml的开源库有很多，在此我就不一一列举了，今天主要说下Rapidxml，我使用这个库也并不是很多，如有错误之处还望大家能够之处，谢谢。
附：
官方链接：http://rapidxml.sourceforge.net/
官方手册：http://rapidxml.sourceforge.net/manual.html
之前有一次用到，碰到了个"坑"，当时时间紧迫并未及时查找，今天再次用到这个库，对这样的"坑"不能踩第二次，因此我决定探个究竟。
先写两段示例：
创建xm：
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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声明 auto nodeRoot = doc.allocate_node(rapidxml::node_element, "Root");//创建一个Root节点 nodeRoot->append_node(doc.allocate_node(rapidxml::node_comment, NULL, "编程语言"));//添加一个注释内容到Root，注释没有name 所以第二个参数为NULL auto nodeLangrage = doc.allocate_node(rapidxml::node_element, "language", "This is C language");//创建一个language节点 nodeLangrage->append_attribute(doc.allocate_attribute("name", "C"));//添加一个name属性到language nodeRoot->append_node(nodeLangrage); //添加一个language到Root节点 nodeLangrage = doc.allocate_node(rapidxml::node_element, "language", "This is C++ language");//创建一个language节点 nodeLangrage->append_attribute(doc.allocate_attribute("name", "C++"));//添加一个name属性到language nodeRoot->append_node(nodeLangrage); //添加一个language到Root节点 doc.append_node(nodeRoot);//添加Root节点到Document std::string buffer; rapidxml::print(std::back_inserter(buffer), doc, 0); std::ofstream outFile("language.xml"); outFile << buffer; outFile.close();} |
结果：
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<?xml version="1.0" encoding="UTF-8"?><Root> <!--编程语言--> <language name="C">This is C language</language> <language name="C++">This is C++ language</language></Root> |
修改xml：
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void MotifyXml(){ rapidxml::file<> requestFile("language.xml");//从文件加载xml rapidxml::xml_document<> doc; doc.parse<0>(requestFile.data());//解析xml auto nodeRoot = doc.first_node();//获取第一个节点，也就是Root节点 auto nodeLanguage = nodeRoot->first_node("language");//获取Root下第一个language节点 nodeLanguage->first_attribute("name")->value("Motify C");//修改language节点的name属性为 Motify C std::string buffer; rapidxml::print(std::back_inserter(buffer), doc, 0); std::ofstream outFile("MotifyLanguage.xml"); outFile << buffer; outFile.close();} |
结果：
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<Root> <language name="Motify C">This is C language</language> <language name="C++">This is C++ language</language></Root> |
由第二个结果得出：
第一个language的name属性确实改成我们所期望的值了，不过不难发现xml的声明和注释都消失了。是怎么回事呢？这个问题也困扰了我一段时间，既然是开源库，那我们跟一下看看他都干了什么，从代码可以看出可疑的地方主要有两处：print和parse，这两个函数均需要提供一个flag，这个flag到底都干了什么呢，从官方给的教程来看 均使用的0，既然最终执行的是print我们就从print开始调试跟踪吧
找到了找到print调用的地方：
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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); } |
继续跟踪：
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// 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; } |
跟进print_children 发现这实际是个递归，我们继续跟踪
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// 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)) ...//省略部分代码 return out;} |
我们发现第8行有一个&判断 查看print_no_indenting的定义：
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// Printing flagsconst int print_no_indenting = 0x1; //!< Printer flag instructing the printer to suppress indenting of XML. See print() function. |
据此我们就可以分析了，按照开发风格统一的思想，parse也应该有相同的标志定义
省略分析parse流程..
我也顺便去查看了官方文档，确实和我预想的一样，贴一下头文件中对这些标志的描述，详细信息可参考官方文档
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// 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; |
根据以上提供的信息我们改下之前的源代码：
将
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doc.parse<0>(requestFile.data());//解析xmlauto nodeRoot = doc.first_node("");//获取第一个节点，也就是Root节点 |
改为
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doc.parse<rapidxml::parse_declaration_node | rapidxml::parse_comment_nodes | rapidxml::parse_non_destructive>(requestFile.data());//解析xmlauto nodeRoot = doc.first_node("Root");//获取第一个节点，也就是Root节点 |
这里解释一下，parse加入了三个标志，分别是告诉解析器创建声明节点、告诉解析器创建注释节点、和不希望解析器修改传进去的数据，第二句是当有xml的声明时，默认的first_node并不是我们期望的Root节点，因此通过传节点名来找到我们需要的节点。
注：
1、这个库在append的时候并不去判断添加项（节点、属性等）是否存在
2、循环遍历时对项（节点、属性等）进行修改会导致迭代失效
总结：用别人写的库，总会有些意想不到的问题，至今我只遇到了这些问题，如果还有其它问题欢迎补充，顺便解释下"坑"并不一定是用的开源库有问题，更多的时候可能是还没有熟练的去使用这个工具。
感谢rapidxml的作者，为我们提供一个如此高效便利的工具。
以上这篇浅谈使用Rapidxml 库遇到的问题和分析过程(分享)就是小编分享给大家的全部内容了，希望能给大家一个参考，也希望大家多多支持服务器之家。
