#include "../rapidxml/rapidxml.hpp"
#include <fstream>

int main()
{
    rapidxml::xml_document<> doc;
    std::ofstream file("text.xml");
    file << doc;
}

||=== Build: Debug in sample (compiler: GNU GCC Compiler) ===|
/src/main.cpp||In function 'int main()':|
/src/main.cpp|8|error: cannot bind 'std::basic_ostream<char>' lvalue to 'std::basic_ostream<char>&&'|
/usr/include/c++/4.9.2/ostream|602|note: initializing argument 1 of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = rapidxml::xml_document<char>]'|
||=== Build failed: 1 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

Codeblocks zeigt auf diesen Teil von ostream:

#if __cplusplus >= 201103L
  /**
   *  @brief  Generic inserter for rvalue stream
   *  @param  __os  An input stream.
   *  @param  __x  A reference to the object being inserted.
   *  @return  os
   *
   *  This is just a forwarding function to allow insertion to
   *  rvalue streams since they won't bind to the inserter functions
   *  that take an lvalue reference.
  */
  template<typename _CharT, typename _Traits, typename _Tp>
    inline basic_ostream<_CharT, _Traits>&
->    operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
    { return (__os << __x); }
#endif // C++11

Ich verstehe den Fehler nicht, in jedem Beispiel von RapidXml wird das Dokument so gespeichert. Kann doc nicht in eine rvalue umgewandelt werden?

Use print() function or operator <<, which are defined in rapidxml_print.hpp header.