double.cpp

#include <iostream>
#include <ios>
#include <iomanip>
#include <cstring>
#include <cstdlib>

int main(int argc, char **argv)
{
    if (argc != 2) {
        std::cerr << "Usage: " << (argc ? argv[0] : "double") << " <number>\n";
        return 0;
    }

    char *end = nullptr;
    double d = std::strtod(argv[1], &end);
    if (d == 0.0 && end == argv[1]) {
        std::cerr << "Invalid number: " << argv[1] << '\n';
        return 0;
    }

    auto *buf = reinterpret_cast<unsigned char*>(&d);
    std::cout << std::hex << std::setfill('0');
    for (std::size_t i = 0; i < sizeof(double); ++i) {
        std::cout << std::setw(2) << static_cast<unsigned>(buf[i]) << ' ';
    }
    std::cout << '\n';
}