std::string формата hex в unsigned char и обратно.

std::string data = "000110";

unsigned char* payload;
payload[0] = 0x00;
payload[1] = 0x01;
payload[2] = 0x10;

    for(int i=0;i<data.size();i+=2)
    {
        payload[i/2]=(data[i]-'0')*16+(data[i+1]-'0');
    }

void hex2uchar(std::string data, unsigned char* payload) {
    std::stringstream convertStream;
    for (int offset = 0, i = 0; offset < data.length(); offset += 2, i++) {
        unsigned int buffer;
 
        convertStream << std::hex << data.substr(offset, 2);
        convertStream >> std::hex >> buffer;
 
        payload[i] = static_cast<unsigned char>(buffer);
 
        convertStream.str(std::string());
        convertStream.clear();
    }
}
 
std::string data = "000110";
unsigned char payload[3];
hex2uchar(data, payload);
printf("Result:\r\n");
for (int i = 0; i < data.length() / 2; i++) {
    printf("%02X | ", payload[i]);
}
printf("\r\n");

unsigned char hex2uchar(char data) {
    if (data >= '0' && data <= '9')
        return data - '0';
    else if (data >= 'a' && data <= 'f')
        return 10 + data - 'a';
    else
        return 10 + data - 'A';
}
 
void hex2uchar(std::string data, unsigned char* payload) {
    for (int i = 0; i < data.size(); i += 2) {
        payload[i / 2] = hex2uchar(data[i]) * 16 + hex2uchar(data[i + 1]);
}

#include <iostream>
#include <iomanip>
 
unsigned char GetHex(char ch)
{
    if (ch >= '0' && ch <= '9')
        return ch & 0x0f;
    else
    {
        ch = toupper(ch);
        if (ch >= 'A' && ch <= 'F')
            return toupper(ch) - 'A' + 10;
        else
            return 0xff;
    }
}
 
unsigned char* str2hex(std::string str, size_t &iRes)
{
    size_t len = str.length();
    unsigned char* res = new unsigned char[(len + 1) / 2];
    iRes = 0;
    size_t iStr = 0;
    if (len & 1)
    {
        res[iRes++] = GetHex(str[iStr++]);
        --len;
    }
    while (len)
    {
        res[iRes++] = (GetHex(str[iStr++]) << 4) + GetHex(str[iStr++]);
        len -= 2;
    }
    return res;
}
 
int main()
{
    std::string data = "0001e0";
    size_t len;
    unsigned char* r = str2hex(data, len);
    for (size_t i = 0; i < len; ++i)
        std::cout << std::setfill('0') << std::setw(2) << std::hex << (unsigned int)r[i] << ' ';
}

#include <iostream>
#include <vector>
#include <charconv>
#include <iomanip>
using namespace std;
int main()
{
    string src = "000110aff0ff0e";
    vector<unsigned char> res(src.size()/2 + src.size()%2);
    const char * first = &src[0];
    const char * const last  = first + src.size();
    size_t i = 0;
    while(first < last){
        from_chars(first, first+2 > last ? first + 1 : first + 2 , res[i++], 16);
        first += 2;
    }
    
    for (auto v : res){
        cout << hex << setw(2) << setfill('0') << int(v) << ' ';
    }
    
    return 0;
}