c++ - Operator overload for [] operator -

why need overload [] operator? have never come across practical scenario necessary. can tell me practical use case this.

err.. std::vector<t>, std::basic_string<t>, std::map<k, v>, , std::deque<t> ?

i used class representing registry key, operator[] returned object representing registry value string between []s.

see also, spirit parser framework, uses [] semantic actions.