: boost::spirit::closure<vars, int> // CRTP

{

member1 value; // ...called "value" in lazy expressions

: public grammar<calculator, vars::context_t> // CRTP

{

template <class Tokenizer>

struct definition

{

subrule<0, vars::context_t> expression;

subrule<1, vars::context_t> group;

subrule<2, vars::context_t> factor;

subrule<3, vars::context_t> term;

subrule<4, vars::context_t> integer;

rule<Tokenizer> top;

definition(calculator const& self)

{

top = (

expression =

term[expression.value = arg1]

>> *( ('+' >> term[expression.value += arg1])

|('-' >> term[expression.value -= arg1]) )

, group =

'(' >> expression[group.value = arg1] >> ')'

, factor =

integer[factor.value = arg1]

| group [factor.value = arg1]

, term =

factor[term.value = arg1]

>> *( ('*' >> factor[term.value *= arg1])

|('/' >> factor[term.value /= arg1]) )

, integer =

int_p[integer.value = arg1]

)[ self.value = arg1 ];

}

// tell Spirit to start parsing with "top"

rule<Tokenizer> const& start() const { return top; }

};

{

calculator calc; // our grammar

std::string str;

while (std::getline(std::cin, str))

{

int n = 0;

parse(str.c_str(), calc[var(n) = arg1], space_p);

std::cout << "result = " << n << std::endl;

}