> 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

> 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 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"; }