Converting a hex string to a byte array

Question

What is the best way to convert a variable length hex string e.g. \"01A1\" to a byte array containing that data.

i.e converting this:

st         


        

Answer 1

So for fun, I was curious if I could do this kind of conversion at compile-time. It doesn't have a lot of error checking and was done in VS2015, which doesn't support C++14 constexpr functions yet (thus how HexCharToInt looks). It takes a c-string array, converts pairs of characters into a single byte and expands those bytes into a uniform initialization list used to initialize the T type provided as a template parameter. T could be replaced with something like std::array to automatically return an array.

#include 
#include 
#include 
#include 

/* Quick and dirty conversion from a single character to its hex equivelent */
constexpr std::uint8_t HexCharToInt(char Input)
{
    return
    ((Input >= 'a') && (Input <= 'f'))
    ? (Input - 87)
    : ((Input >= 'A') && (Input <= 'F'))
    ? (Input - 55)
    : ((Input >= '0') && (Input <= '9'))
    ? (Input - 48)
    : throw std::exception{};
}

/* Position the characters into the appropriate nibble */
constexpr std::uint8_t HexChar(char High, char Low)
{
    return (HexCharToInt(High) << 4) | (HexCharToInt(Low));
}

/* Adapter that performs sets of 2 characters into a single byte and combine the results into a uniform initialization list used to initialize T */
template 
constexpr T HexString(const char (&Input)[Length], const std::index_sequence&)
{
    return T{HexChar(Input[(Index * 2)], Input[((Index * 2) + 1)])...};
}

/* Entry function */
template 
constexpr T HexString(const char (&Input)[Length])
{
    return HexString(Input, std::make_index_sequence<(Length / 2)>{});
}

constexpr auto Y = KS::Utility::HexString>("ABCDEF");