用boost::spirit实现的表达式求值

用boost::spirit实现一个表达式求值看上去比较简单。我这个还有点问题，有空格时会解析失败，请大家看看是什么原因？

123*34+2323/324
parsing succeeded
result = 4189
2 + 3
parsing failed

#include <iostream>
#include <stack>
#include <functional>

#include <boost/function.hpp>
// #define BOOST_SPIRIT_DEBUG
#include <boost/spirit.hpp>

using namespace std;
stack<int> evaluationStack;

struct Push
{
void operator()(int d) const
{
evaluationStack.push(d);
}
};

void doOp(boost::function<int(int, int)> op)
{
int rhs = evaluationStack.top();
evaluationStack.pop();
int lhs = evaluationStack.top();
evaluationStack.pop();
int result = op(lhs, rhs);
evaluationStack.push(result);
}

void add2(char const*, char const*) { doOp(std::plus<int>()); }
void sub2(char const*, char const*) { doOp(std::minus<int>()); }
void mul2(char const*, char const*) { doOp(std::multiplies<int>()); }
void div2(char const*, char const*) { doOp(std::divides<int>()); }

int main()
{
using namespace boost::spirit;

/*
group ::= '(' expression ')'
factor ::= integer | group
term ::= factor (('*' factor) | ('/' factor))*
expression ::= term (('+' term) | ('-' term))*

Spirit:
group = '(' >> expression >> ')';
factor = integer | group;
term = factor >> *(('*' >> factor) | ('/' >> factor));
expression = term >> *(('+' >> term) | ('-' >> term));
*/

rule<> group, factor, term, expression;

group = '(' >> expression >> ')';
factor = int_p[Push()] | group;
term = factor
>> *( ('*' >> factor) [&mul2]
| ('/' >> factor) [&div2] )
;
expression = term
>> *( ('+' >> term) [&add2]
| ('-' >> term) [&sub2] )
;

string s;
while (getline(cin, s))
{
if (parse(s.c_str(), expression, space_p).full)
{
cout << "parsing succeeded\n";
cout << "result = " << evaluationStack.top();
evaluationStack.pop();
}
else
{
cout << "parsing failed";
}
cout << endl;
}
}