Perhaps a more experienced user knows, but I don't think that's available through the website's GUI.
I would just use Google or a similar search engine, and search:
site:cplusplus.com "TheUserName" key words or phrases
The follow-up question is, why would you want to see every post? Who would want to look through 55 pages of posts? I would type in their name with a few keywords to narrow your search to something you're actually interested in.