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:

std::string = "01A1"; 

into this

char* hexArray; int hexLength; 

or this

std::vector<char> hexArray; 

so that when I write this to a file and hexdump -C it I get the binary data containing 01A1.

Answer 1

This implementation uses the built-in strtol function to handle the actual conversion from text to bytes, but will work for any even-length hex string.

std::vector<char> HexToBytes(const std::string& hex) {   std::vector<char> bytes;    for (unsigned int i = 0; i < hex.length(); i += 2) {     std::string byteString = hex.substr(i, 2);     char byte = (char) strtol(byteString.c_str(), NULL, 16);     bytes.push_back(byte);   }    return bytes; } 

Answer 2

This ought to work:

int char2int(char input) {   if(input >= '0' && input <= '9')     return input - '0';   if(input >= 'A' && input <= 'F')     return input - 'A' + 10;   if(input >= 'a' && input <= 'f')     return input - 'a' + 10;   throw std::invalid_argument("Invalid input string"); }  // This function assumes src to be a zero terminated sanitized string with // an even number of [0-9a-f] characters, and target to be sufficiently large void hex2bin(const char* src, char* target) {   while(*src && src[1])   {     *(target++) = char2int(*src)*16 + char2int(src[1]);     src += 2;   } } 

Depending on your specific platform there's probably also a standard implementation though.