The symbian source code is available. Looks like it uses a class-specific operator new() overload.

https://github.com/SymbianSource/oss.FCL.sf.os.kernelhwsrv/b...

2. Initialisation of the CBase derived object to binary zeroes through a specific CBase::operator new() - this means that members, whose initial value should be zero, do not have to be initialised in the constructor. This allows safe destruction of a partially-constructed object.