: 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

{

// all our rules have an attached int called "value," too...

rule<Tokenizer, vars::context_t>

expression, term, factor, group, integer;

// ...except the top rule

rule<Tokenizer> top;

// build the grammar

definition(calculator const& self)

{

top = expression[self.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])

)

;

expression = term[expression.value = arg1]

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

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

)

;

integer = int_p[integer.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;

}