"0123456789abcdef" - > 0x12 0x34 0x56 0x78 ...

stringtohexarray.cpp

#include <cstring>
#include <cstdlib>
#include <cassert>
#include <string>
#include <vector>
#include <stdexcept>
#include <iostream>

int chartoint(char c) {
  const char* buf = "0123456789abcdef";
  const char* ptr = std::strchr(buf, std::tolower(c));

  if (!ptr)
    throw std::runtime_error("Not a hex digit:" + c);
  return ptr - buf;
}

void cast(const char* arr, std::vector<unsigned char>* chvec) {
  size_t arrlen = strlen(arr);
  if (arrlen & 1) {
  	throw std::runtime_error(std::string("odd string length:") + arr);
  }

  while (*arr) {
    int val = chartoint(*arr++) << 4;
    val += chartoint(*arr++);
    chvec->push_back(val);
  }
}

void test() {
  const char* arr = "DD2B23B62AC245DA"; 
  char buf[100];
  std::vector<unsigned char> vec;
  cast(arr, &vec);
  for (auto ch : vec) printf("%02x ", ch);
}
int main() {
  test();
  return 0;
}