Unresolved externals

[jpn]

Where should I put it, can I put it in main.cpp so I can initialize database at app beginning?

An initialization of a static member is needed outside the class, in the global scope. I would strongly advise to put the definition (initialization) into cdatabaseconnection.cpp. That's where it belongs, doesn't it? You can access the pointer and initialize the actual connection anywhere you want by just including the header.