Перевод hex в char

#include <iostream>
#include <vector>
 
char string_to_hex(const char& hex)
{
    char c = hex;
    if(( hex >= '0')&&(hex <= '9')){
        c = hex - '0';
        return c;}
    if(( hex >= 'A')&&(hex <= 'F')){
        c = hex - 'A' + 10;
        return c;}
    if(( hex >= 'a')&&(hex <= 'f')){
        c = hex - 'a' + 10;
        return c;}
}
 
std::vector <char> converter (const std::string& str)
{
    std::vector <char> result;
    result.reserve(str.size() / 2);
    for (int i = 0; i < str.size()/2; ++i)
        result.push_back(((string_to_hex(str[2*i])) << 4) | string_to_hex(str[2*i + 1]));
 
    return result;
}
 
int main()
{
    std::string str = "e8";
    std::vector<char> result = converter(str);
    for (int i = 0; i < result.size(); ++i)
        std::cout << static_cast<unsigned int>(result[i])<< std::endl;
 
    system("pause");
}

std::string sourceStrByte = "e8";               
unsigned char result = (unsigned char) strtol(sourceStrByte.c_str(), NULL, 16);

std::string result = std::string(1, str[2*i]) + std::string(1, str[2*i + 1]);

std::string bin = std::string(&str[2*i], &str[2*i] + 2);

std::string bin = {str[2*i], str[2*i + 1]};

std::string bin(&str[2*i], &str[2*i] + 2);

std::string(str, 2*i, 2);

std::string result = std::string( &str[2*i], 2 );