Printing zero-padded hex with std::cout [duplicate]

Question

This question already has answers here:

How can I print 0x0a instead of 0xa using cout? (9 answers)

Closed 5 years ago.

Say I have a dword I want to output in hex with std::cout and left-pad with zeros, so 0xabcd will be shown as 0x0000abcd. It seems like you would have to do this:

uint32_t my_int = 0xabcd;

std::cout << "0x" << std::hex << std::setw(8) << std::setfill('0')
    << my_int << std::endl;

This seems ridiculous for something that can be accomplished in C with printf("0x%08X\n", my_int);. Is there any way to make this shorter while still using std::cout for output (besides using namespace std)?

Answer 1

I suppose you can write a "stream manipulator". This is useful if you have multiple hex numbers you want to print in this format. This is clearly not an ideal solution, but using a wrapper type you can make your own "format flag" to toggle it. See Sticky custom stream manipulator for more information.

#include <iostream>
#include <iomanip>

static int const index = std::ios_base::xalloc();

std::ostream& hexify(std::ostream& stream) {
    stream.iword(index) = 1;
    return stream;
}

std::ostream& nohexify(std::ostream& stream) {
    stream.iword(index) = 0;
    return stream;
}

struct WrapperType {
    uint32_t _m;
public:
    WrapperType(uint32_t m) : _m(m)
    {
    }

    uint32_t getm() const
    {
        return _m;
    }
};
std::ostream& operator<< (std::ostream& os, const WrapperType& t) {
    if (os.iword(index))
        return os << "0x" << std::hex << std::setw(8) << std::setfill('0') << t.getm();
    else
        return os << t.getm();
}

int main()
{
    WrapperType my_int{0xabcd};
    std::cout << hexify << my_int << my_int;
    std::cout << nohexify << my_int;
}

Answer 2

I would not change the (global) flags of a stream, just a manipulator:

#include <iostream>
#include <iomanip>
#include <limits>

template <typename T>
struct Hex
{
    // C++11:
    // static constexpr int Width = (std::numeric_limits<T>::digits + 1) / 4;
    // Otherwise:
    enum { Width = (std::numeric_limits<T>::digits + 1) / 4 };
    const T& value;
    const int width;

    Hex(const T& value, int width = Width)
    : value(value), width(width)
    {}

    void write(std::ostream& stream) const {
        if(std::numeric_limits<T>::radix != 2) stream << value;
        else {
            std::ios_base::fmtflags flags = stream.setf(
                std::ios_base::hex, std::ios_base::basefield);
            char fill = stream.fill('0');
            stream << "0x" << std::setw(width) << value;
            stream.fill(fill);
            stream.setf(flags, std::ios_base::basefield);
        }
    }
};

template <typename T>
inline Hex<T> hex(const T& value, int width = Hex<T>::Width) {
    return Hex<T>(value, width);
}

template <typename T>
inline std::ostream& operator << (std::ostream& stream, const Hex<T>& value) {
    value.write(stream);
    return stream;
}

int main() {
    std::uint8_t u8 = 1;
    std::uint16_t u16 = 1;
    std::uint32_t u32 = 1;
    std::cout << hex(unsigned(u8), 2) << ", " << hex(u16) << ", " << hex(u32) << '\n';
}

Answer 3

My C++ is rusty, but how about using Boost formatting: http://www.boost.org/doc/libs/1_37_0/libs/format/index.html