Question

What is vftable in high programming languages?

I read something like it’s the address of a virtual object structure, but this is a pretty messy information

Can someone please explain it?

Answer 1

It most likely stands for “Virtual Function Table“, and is a mechanism used by some runtime implementations in order to allow virtual function dispatch.

Mainstream C++ implementations (GCC, Clang, MSVS) call it the vtable. C has no polymorphism. I could only speculate about other languages.

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Here’s what Wikipedia says on the topic:

An object’s dispatch table will contain the addresses of the object’s
dynamically bound methods. Method calls are performed by fetching the
method’s address from the object’s dispatch table. The dispatch table
is the same for all objects belonging to the same class, and is
therefore typically shared between them. Objects belonging to
type-compatible classes (for example siblings in an inheritance
hierarchy) will have dispatch tables with the same layout: the address
of a given method will appear at the same offset for all
type-compatible classes. Thus, fetching the method’s address from a
given dispatch table offset will get the method corresponding to the
object’s actual class.^[1]

The C++ standards do not mandate exactly how dynamic dispatch must be
implemented, but compilers generally use minor variations on the same
basic model.

Typically, the compiler creates a separate vtable for each class. When
an object is created, a pointer to this vtable, called the virtual
table pointer, vpointer or VPTR, is added as a hidden member of this
object (becoming its first member unless it’s made the last^[2]). The
compiler also generates “hidden” code in the constructor of each class
to initialize the vpointers of its objects to the address of the
corresponding vtable. Note that the location of the vpointer in the
object instance is not standard among all compilers, and relying on
the position may result in unportable code. For example, g++
previously placed the vpointer at the end of the object.^[3]

1. Ellis & Stroustrup 1990, pp. 227–232
2. Heading “Multiple Inheritance”
3. CodeSourcery C++ ABI