vptr

Vptr is an internal compiler-generated member used to support dynamic dispatch of virtual functions in polymorphic objects, most commonly in C++. It refers to a hidden pointer inside an object that points to a vtable, a per-class table of function pointers implementing the virtual function calls for that class.

When an object of a class with virtual functions is created, the object typically contains a vptr that initially points to the vtable corresponding to the object’s dynamic type. When a virtual function is called through a base-class reference or pointer, the program uses the vptr to locate the correct function in the vtable and invokes it. The vtable entries correspond to the class’s virtual functions, and the exact layout of the table is implementation-defined.

Multiple inheritance can complicate vptr usage. An object may contain more than one vptr (one for each base subobject that contributes virtual functions), and the compiler may insert pointer adjustments so that calls resolve correctly even when accessed via different base subobjects. Some aspects, such as covariant return types and RTTI integration, influence vtable contents and the mechanics of dispatch, but the precise arrangement is determined by the compiler and ABI.

Portability and standards considerations are important. The C++ standard defines the semantics of virtual functions but does not mandate or specify the use of vptr or vtable, their layout, or their accessibility. Therefore, code should not rely on the existence or structure of vptr for correctness or portability, and debugging or binary compatibility concerns should treat vptr as an implementation detail.