Base64 isn't encryption: it is translation, and is so easy to reverse that it's doesn't count as encryption in any shape, form, or manner.
If you want to convert text to Base64 it's pretty trivial as a simple google will show:
Base64 C++ - Google Search[
^]
It you want to encrypt it, you need to be a lot more sophisticated and look at C++ encryption services:
C++ encryption services - Google Search[
^]