Simple algorithm to convert a hexadecimal string to a base64 string.

hex_to_base64.cpp

#include <iostream>
#include <map>
#include <string>
#include <bitset>

std::string hex_to_bin(std::string hex_str); 
std::string bin_to_base64(std::string bin_str);
std::string hex_to_base64(std::string hex_str);

int main()
{
	std::string hex_str;
	std::cin >> hex_str;


	std::cout << hex_to_base64(hex_str) << '\n';

}

std::string hex_to_base64(std::string hex_str)
{
	std::string bin_str = hex_to_bin(hex_str);

	return bin_to_base64(bin_str);
}

std::string hex_to_bin(std::string hex_str)
{
	std::map<char,int> hex_table{ {'0',0}, {'1',1}, {'2',2}, {'3',3}, {'4',4}, {'5',5}, {'6',6}, {'7',7}, {'8',8}, 
				      {'9',9}, {'a',10}, {'b',11}, {'c',12}, {'d',13}, {'e',14}, {'f',15}};

	std::string bit_str;

	for(char hex_char  : hex_str)
		bit_str += std::bitset<4>(hex_table[hex_char]).to_string();
	
	

	return bit_str;
}

std::string bin_to_base64(std::string bin_str)
{
	char base64_table[] = {65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90,
			 	97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 
				119, 120, 121, 122, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 43, 47};

	std::string base64_str;

	auto bit = begin(bin_str);

	while( bit != end(bin_str) ) 
	{
		std::string base64_part_str;
		int bit_counter{6};

		while(bit_counter-- && bit != end(bin_str) )
		{
			base64_part_str += *bit++;
		}

		int index = std::stoi(base64_part_str,nullptr,2); 

		base64_str += base64_table[index];
		

	}
	


	return base64_str;
	
	
}