This is particularly sucky to solve in C and C++ because you don't get arbitrary precision literals.
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/lexical_cast.hpp>
#include <iostream>
using fl50 = boost::multiprecision::cpp_dec_float_50;
int main() {
auto a = boost::lexical_cast<fl50>("9999999999999999.7");
auto b = boost::lexical_cast<fl50>("9999999999999998.5");
std::cout << (a - b) << "\n";
}
works
int main() {
fl50 a = 9999999999999999.7;
fl50 b = 9999999999999998.5;
std::cout << (a - b) << "\n";
}
doesn't, even if you change fl50 out for a quad precision binary float type.
> Even user-defined literals in C++11 and later don't let you express custom floating point expressions
Note that in your code sample you're not actually using user-defined literals (https://en.cppreference.com/w/cpp/language/user_literal). This works (based on on your earlier code sample and adding user-defined literals):
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/lexical_cast.hpp>
#include <iostream>
using fl50 = boost::multiprecision::cpp_dec_float_50;
fl50 operator"" _w(const char* s) { return boost::lexical_cast<fl50>(s); }
int main() {
fl50 a = 9999999999999999.7_w;
fl50 b = 9999999999999998.5_w;
std::cout << (a - b) << "\n";
}
