Simply, copy base64.hpp and base64.cpp in your project's directory and include base64.hpp. To encode a std::string in base64 use Base64Encode(std::string str). To decode a base64 encoded std::string use Base64Decode(std::string str).
#include <iostream>
#include "base64.hpp"
int main()
{
std::string str;
std::cout << "Enter string to encode to base64: ";
std::cin >> str;
std::string b64 = Base64Encode(str);
std::cout << "Base64: " << b64 << std::endl;
std::cout<<"Now decoding: "<<Base64Decode(b64)<<std::endl;
return 0;
}