This is very old, but most unknown container type (so if fits into C++ iceberg): [std::valarray](https://en.cppreference.com/w/cpp/numeric/valarray). It has strange [operator\[\]](https://en.cppreference.com/w/cpp/numeric/valarray/operator_at).
This is very old, but most unknown container type (so if fits into C++ iceberg):
std::valarray.
It has strange operator[].