Mechanics of virtual dispatch with vtables

How do C++ compilers typically implement virtual function calls?

With a virtual table per class and a hidden pointer per object. Each polymorphic class has a table of function pointers to its overrides, and each object stores a vptr to its class's table, set by the constructors. A virtual call loads the vptr, indexes the slot for that function and makes an indirect call, costing an extra pointer per object and an indirection per call.