error LNK2019: unresolved external symbol “public: __thiscall RNG::RNG(unsigned __int64)” (??0RNG@@QAE@_K@Z) referenced in function _main

Question

I\'m getting the above error when I build the files below, using cl.exe in VSCode. I\'ve been staring at this code for a full day and can\'t see the error. I\'ve tried a bunch o