#include <iostream>
#include <boost/xpressive/xpressive.hpp>
#include <boost/xpressive/regex_actions.hpp>
#include <map>

using namespace boost::xpressive;

int main()
{
	std::map<std::string, int> result;
	std::string str("aaa=>1 bbb=>23 ccc=>456");

	// Match a word and an integer, separated by =>,
	// and then stuff the result into a std::map<>
	sregex pair = ( (s1= +_w) >> "=>" >> (s2= +_d) )[ ref(result)[s1] = as<int>(s2) ];

	// Match one or more word/integer pairs, separated
	// by whitespace.
	sregex rx = pair >> *(+_s >> pair);

	if(regex_match(str, rx))
	{
		std::cout << result["aaa"] << '\n';
		std::cout << result["bbb"] << '\n';
		std::cout << result["ccc"] << '\n';
	}

	return 0;
}

The complete error:
D:\Projekte\xpressive-test\main.cpp:29: Fehler:C2668: 'boost::xpressive::ref' : ambiguous call to overloaded function
D:\Frameworks\boost\boost_147\boost/xpressive/regex_actions.hpp(850): could be 'const boost::xpressive::reference<T> boost::xpressive::ref<std::map<_Kty,_Ty>>(T &)'
with
[
    T=std::map<std::string,int>,
    _Kty=std::string,
    _Ty=int
]
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\INCLUDE\xrefwrap(439): or       'std::tr1::reference_wrapper<_Ty> std::tr1::ref<std::map<_Kty,int>>(_Ty &)' [found using argument-dependent lookup]
with
[
    _Ty=std::map<std::string,int>,
    _Kty=std::string
]
while trying to match the argument list '(std::map<_Kty,_Ty>)'
with
[
    _Kty=std::string,
    _Ty=int
]

What's the problem here?

Thank you very much!