Try yourself :)

#include <string> 
#include <iostream> 
using namespace std;
string base64_decode(string const& s);
static const string base64_chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
static inline bool is_base64(unsigned char c) {return (isalnum(c) || (c == '+') || (c == '/'));}
string base64_decode(string const& encoded_string) {size_t in_len = encoded_string.size(), i=0, j=0;int in_;unsigned char char_array_4[4], char_array_3[3];string ret;while (in_len-- && ( encoded_string[in_] != '=') && is_base64(encoded_string[in_])) {char_array_4[i++] = encoded_string[in_]; in_++;if (i ==4) {for (i = 0; i <4; i++)char_array_4[i] = static_cast<unsigned char>(base64_chars.find(char_array_4[i]));char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];for (i = 0; (i < 3); i++)ret += char_array_3[i];i = 0;}}if (i) {for (j = i; j <4; j++)char_array_4[j] = 0;for (j = 0; j <4; j++)char_array_4[j] = static_cast<unsigned char>(base64_chars.find(char_array_4[j]));char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];for (j = 0; (j < i - 1); j++) ret += char_array_3[j];}return ret;}
int main() {string b = "6YGH6KeB5L2g77yM5oiR5L6/5LiN5ZCO5oKU6L+H5Y6755qE5LiA5YiH44CC5bCP54eV77yM5oiR5Zac5qyi5L2g5ZGA";cout << base64_decode(b) << endl;return 0;}