Parsing float followed by string containing "e" character - c++

Question

I'm trying to parse this type of string
1.2e3ex
1.2e3 ex
And have set up
x3::float_ >> "ex"
Unfortunately, this fails to parse
1ex
Full example code:
#include <iostream>
#include <boost/spirit/home/x3.hpp>
namespace x3 = boost::spirit::x3;
const auto parser = x3::float_ >> "em";
int main()
{
std::string input = "1em";
auto first = input.begin();
auto last = input.end();
float value{};
bool result = x3::phrase_parse(first, last, parser, x3::blank, value);
if(result)
{
if(first == last)
std::cout << "parse succesful: " << value << '\n';
else
std::cout << "incomplete parse: " << value << '\n';
}
else
std::cout << "parse unsuccesful\n";
}
Available live on Coliru as well.
It seems I'd need to jump through some hoops,
struct non_scientific_float_policy : x3::real_policies<float>
{
template <typename Iterator>
static bool parse_exp(Iterator& first, Iterator const& last)
{
return false;
}
};
const auto non_scientific_float = x3::real_parser<float, non_scientific_float_policy>{};
and provide an alternative:
const auto parser = non_scientific_float >> "em" | x3::float_ >> "em";
Is there no other way?

Answer 1

You can solve the issue by tuning the real policy parse_exp in such way that exponent detection must expect not only [eE] character but [eE][-+]?[0-9].
#include <iostream>
#include <boost/spirit/home/x3.hpp>
namespace x3 = boost::spirit::x3;
template <typename T>
struct alt_real_policies : x3::real_policies<T>
{
template <typename Iterator>
static bool parse_exp(Iterator& first, Iterator const& last)
{
Iterator save = first;
if (x3::real_policies<T>::parse_exp(first, last)) {
Iterator iter = first;
if (x3::extract_int<x3::unused_type, 10, 1, 1>::call(iter, last, x3::unused))
return true;
}
first = save;
return false;
}
};
const x3::real_parser<float, alt_real_policies<float>> altfloat;
const auto parser = altfloat >> "em";
int main()
{
std::string input = "1em";
auto first = input.begin();
auto last = input.end();
float value{};
bool result = x3::phrase_parse(first, last, parser, x3::blank, value);
if (result)
{
if (first == last)
std::cout << "parse succesful: " << value << '\n';
else
std::cout << "incomplete parse: " << value << '\n';
}
else
std::cout << "parse unsuccesful\n";
}
http://coliru.stacked-crooked.com/a/f60f334c960cb602

Answer 2

You can use an alternative policy for non-greedy parsing of the exponent. The very simplest I can think of is:
Live On Coliru
#include <boost/spirit/home/x3.hpp>
#include <iostream>
namespace x3 = boost::spirit::x3;
template <typename T>
struct no_exponent : x3::real_policies<T> {
template <typename It>
static bool parse_exp(It, It) { return false; }
};
x3::real_parser<double, no_exponent<double> > noexp_;
const auto parser = (x3::float_ | noexp_) >> "em";
int main() {
std::string input = "-1.67em";
auto first = input.begin();
auto last = input.end();
float value{};
bool result = x3::phrase_parse(first, last, parser, x3::blank, value);
if (result) {
if (first == last)
std::cout << "parse succesful: " << value << '\n';
else
std::cout << "incomplete parse: " << value << '\n';
} else
{
std::cout << "parse unsuccesful\n";
}
}
Printing:
parse succesful: -1.67