Compiler Explorer

Source code

#include <cerrno>
#include <cmath>
#include <cstdlib>
#include <stdexcept>
#include <string_view>

auto parse_float64(std::string_view s) -> double {
    char* ed = nullptr;
    errno = 0;
    auto v = std::strtod(s.data(), &ed);
    if (errno != 0) {
        auto msg = std::signbit(v) ? "negative" : "positive";
        if (v == 0)
            throw std::underflow_error{msg};
        else
            throw std::overflow_error{msg};
    } else {
        if (ed == std::to_address(s.end()))
            return v;
        else
            throw std::invalid_argument{"extra bytes"};
    }
}

#include <iostream>

int main() {
    using namespace std::literals;
    for (auto flt : {
             "3.14e-400",
             "42.2172",
             "-1.3e1000",
             "5.e1000",
             "-2.4e-1024",
         }) {
        try {
            std::cout << flt << " --> " << parse_float64(flt) << '\n';
        } catch (std::underflow_error& exc) {
            std::cout << "undeflow: " << exc.what() << '\n';
        } catch (std::overflow_error& exc) {
            std::cout << "overflow: " << exc.what() << '\n';
        } catch (std::invalid_argument& exc) {
            std::cout << "failed to parse: " << exc.what() << '\n';
        }
    }
}