error: expected primary-expression before ‘if’

Question

这是我的代码，我不知道'if'附近哪里错了，c中的"primary-expression"是什么？

xml_parser.h

#define xpath(smth, exp) if (sizeof(*smth) == sizeof(xmlNode)) { xpath_node((xml_node_ptr)smth, exp); } else{ xpath_doc((xml_doc_ptr)smth, exp); }

xml_node_ptr *xpath_node(xml_node_ptr n, str exp);

xml_node_ptr *xpath_doc(xml_doc_ptr n, str exp);

xml_parser_test.cpp

TEST_F(XmlParserTest, xpath2) {
    xml_node_ptr n = xpath(d, "/html/body");
    xpath(n, "/div");
}

这是我的输出：

/home/roroco/Dropbox/rbs/ro_sites/c/xml_parser_test.cpp: In member function ‘virtual void XmlParserTest_xpath2_Test::TestBody()’:
/home/roroco/Dropbox/rbs/ro_sites/c/xml_parser.h:17:26: error: expected primary-expression before ‘if’
 #define xpath(smth, exp) if (sizeof(*smth) == sizeof(xmlNode)) { xpath_node((xml_node_ptr)smth, exp); } else{ xpath_doc((xml_doc_ptr)smth, exp); }
                          ^
/home/roroco/Dropbox/rbs/ro_sites/c/xml_parser_test.cpp:26:22: note: in expansion of macro ‘xpath’
     xml_node_ptr n = xpath(d, "/html/body");
                  ^

我的完整来源是 here

Answer 1

您的代码几乎可以扩展到

xml_node_ptr n = if (sizeof(*d) == sizeof(xmlNode)) { xpath_node((xml_node_ptr)d, "/html/body"); } else{ xpath_doc((xml_doc_ptr)d, "/html/body"); }

由预处理器。很明显这是错误的。要回答你的第二个问题， primary-expression 是 C++ 语法中预期的非终结符。主要表达式是算术、函数调用等

error: expected primary-expression before ‘if’

error: expected primary-expression before ‘if’

c++

macros

libxml2