The operation requested here is the opposite of the one implemented in the previous problem. This time, however, we could write a function and not a function template. The input is an std::string_view, which is a lightweight wrapper for a sequence of characters. The output is a vector of 8-bit unsigned integers. The following hexstr_to_bytes function transforms every two text characters into an unsigned char value ("A0" becomes 0xA0), puts them into an std::vector, and returns the vector:
unsigned char hexchar_to_int(char const ch)
{
if (ch >= '0' && ch <= '9') return ch - '0';
if (ch >= 'A' && ch <= 'F') return ch - 'A' + 10;
if (ch >= 'a' && ch <= 'f') return ch - 'a' + 10;
throw std::invalid_argument("Invalid hexadecimal character");
}
std::vector<unsigned char> hexstr_to_bytes(std::string_view str)
{
std::vector<unsigned char> result;
for (size_t i = 0; i < str.size(); i += 2)
{
result.push_back(
(hexchar_to_int(str[i]) << 4) | hexchar_to_int(str[i+1]));
}
return result;
}
This function assumes the input string contains an even number of hexadecimal digits. In cases where the input string contains an odd number of hexadecimal digits, the last one is discarded (so that "BAD" becomes {0xBA}). As a further exercise, modify the preceding function so that, instead of discarding the last odd digit, it considers a leading zero so that "BAD" becomes {0x0B, 0xAD}. Also, as yet another exercise, you can write a version of the function that deserializes content that has the hexadecimal digits separated by a delimiter, such as space (for example "BA AD F0 0D").
The next code sample shows how this function can be used:
int main()
{
std::vector<unsigned char> expected{ 0xBA, 0xAD, 0xF0, 0x0D, 0x42 };
assert(hexstr_to_bytes("BAADF00D42") == expected);
assert(hexstr_to_bytes("BaaDf00d42") == expected);
}