1. Is it allowed to use references and snippets in codeforces official rounds ?

Most of my friends uses references in the contests , so I was wondering if it's allowed to use a code that I wrote before such like segment tree implementation , or it's considered as cheating ?

2. Will using snippets affect on my performance in olympiads and other contests ?

I'm practicing for IOI and ICPC , and I'm afraid that using snippets will make it difficult for me to implement those codes which I used to use snippets for . So will that really affect my performance in olympiads if I used it well ?

Guide me please.. And Thank you