Thanks guys for your help.
Mirek, my intention why I want it as member functions is, because my RPC Methods all need a database connection, so in one class function I open the connection and keep it open. And because I don't want to pass this PostgreSqlSession to all RPC Functions, I want to use a member.
Furthermore I need an Instance of a very big class and don't want to pass this either, so a member seems to be the best solution in my opinion.

PS: Maybe you can explain this code? I mean... it compiles, but I really don't know what this does:

[](RpcData& rpc){Single<MyRpcClass>().Status(rpc);});