Boost Spirit int into string rule - c++

Question

Assuming I have the following rule expecting a string value but should convert it to an integer:
integer %=
attr_cast<int,std::string>(
lexeme[
-(char_('+') | char_('-')) >>
char_("1-9") >> *char_("0-9")
]
)
;
so I defined a transformation structure:
template <>
struct transform_attribute<int, std::string, qi::domain>
{
// input type
typedef std::string& type;
static std::string pre(int & d) {
return "";
}
static void post(int & val, const type attr) {
val = boost::lexical_cast<int>(attr);
}
static void fail(int &) {}
};
unfortunately it expects to convert the int reference of the left rule into a string reference (I removed the reference operator but it does not work).
On the Internet I saw examples working the other way int to string where they don't use references for the int. How can I define a proper transformation customly?

Answer 1

First off, you don't have to write this:
integer %= qi::int_; // Job Done
Next, you could write the qi::attr_cast<std::string>(...) easier and more efficient by doing as_string[ ... ].
Next, if you do want to parse ints the laborious way, try this:
bool ok = parse(f, l,
(matches['-'] | -lit('+') >> attr(false)) [ phx::ref(negative) = _1 ] >> // sign
eps [ _val = 0 ] >>
+digit [ _val *= 10, _val += (_1 - '0') ],
parsed);
See a live demo on Coliru testing for (u)int(8,16,32,64,max)_t on the whole range:
The qi::int_parser<> template used above (via qi::int_) is essentially a generalization of this approach, but more efficient.
Now, you can of course do the traits trick if you insist:
namespace boost { namespace spirit { namespace traits {
template <typename Int>
struct assign_to_attribute_from_value<
Int,
std::string,
typename std::enable_if<std::is_integral<Int>::value, void>::type // Enabler
>
{
static void call(std::string const& val, Int& attr) {
//std::cout << __PRETTY_FUNCTION__ << "('" << val << "')\n";
attr = boost::lexical_cast<Int>(val);
}
};
} } }
Now this would be shooting a fly with a canon. Never mind that boost::lexical_cast doesn't handle uint8_t and int8_t correctly for this purpose (treating them specially as char and unsigned char?), so I had to also hard code exception for these:
// boost lexical_cast does not usefully support `char` types as integrals... (SIC)
template <>
struct assign_to_attribute_from_value<signed char, std::string> {
static void call(std::string const& val, signed char& attr) {
int tmp;
assign_to_attribute_from_value<int, std::string>::call(val, tmp);
attr = static_cast<signed char>(tmp);
}
};
template <>
struct assign_to_attribute_from_value<unsigned char, std::string> {
static void call(std::string const& val, unsigned char& attr) {
unsigned int tmp;
assign_to_attribute_from_value<unsigned int, std::string>::call(val, tmp);
attr = static_cast<unsigned char>(tmp);
}
};
Now all the test cases passed with
Int parsed = 0;
bool ok = parse(f, l, as_string [ -char_("-+") >> +digit ], parsed);
See it Live On Coliru as well.
Now let me conclude with the only "sane" approach: don't reinvent the wheel
Int parsed = 0;
bool ok = qi::parse(f, l, qi::auto_, parsed);
Full Program Live On Coliru
#include <boost/spirit/include/qi.hpp>
template <typename Int>
void do_test() {
for (Int const testcase : { std::numeric_limits<Int>::min(), Int(), std::numeric_limits<Int>::max() }) {
auto const input = std::to_string(testcase);
auto f(input.begin()), l(input.end());
Int parsed = 0;
bool ok = boost::spirit::qi::parse(f, l, boost::spirit::qi::auto_, parsed);
if (!ok || f!=l)
throw std::runtime_error("parse error");
std::cout << std::boolalpha << (testcase==parsed) << "\t" << testcase << " -> " << parsed << "\n";
}
}
int main() {
do_test<int16_t>(); do_test<uint16_t>();
do_test<int32_t>(); do_test<uint32_t>();
do_test<int64_t>(); do_test<uint64_t>();
do_test<intmax_t>(); do_test<uintmax_t>();
}