vptrs

Vptrs, short for virtual table pointers, are a common implementation detail used to support dynamic dispatch in object-oriented languages such as C++. They enable a program to decide at runtime which function to call for an object when a method is marked virtual, ensuring the most-derived override is invoked regardless of the static type of the reference or pointer.

In typical implementations, each polymorphic class has a corresponding vtable, a table of function pointers for its virtual functions. Each object of such a class contains a vptr that points to that class’s vtable. When a virtual method is invoked through a base-class pointer or reference, the program uses the object’s vptr to locate the correct function in the vtable for the dynamic type, and then calls that function.

vptr and vtable are not part of the C++ language standard; they are compiler and ABI specifics. Consequently, their exact layout and rules can vary between compilers. In most cases, constructors ensure the vptr points to the appropriate vtable for the object’s current dynamic type, and virtual destructors help maintain correct destruction through base pointers.

Multiple inheritance can require more than one vptr per object, since different base subobjects may have independent vtables. While vptrs enable efficient dynamic dispatch, they introduce a small memory and initialization overhead and one more layer of indirection. They also influence debugging and binary compatibility, as the exact vtable layout and vptr management are implementation details rather than portable language concepts.