How are Virtual Functions and VTables Implemented in C ?

Unveiling the Intricacies of Virtual Functions and VTables

Virtual functions are a vital concept in C , allowing the implementation of polymorphic behavior. But how are these virtual functions implemented at the core level? Dive into this article to unravel the intricacies of virtual functions and their indispensable companion, the vtable.

How are Virtual Functions Implemented?

When a class defines virtual functions, a vtable (virtual method table) is generated for that class. This vtable stores pointers to the virtual functions. Each object of the class that contains virtual functions contains a vptr (virtual pointer) that points to the base address of the vtable in memory.

When a virtual function is called, the v-table is used to locate the function address. The vptr provides the base address of the vtable, allowing for dynamic binding during runtime.

Vtable Manipulation and Accessibility

Can the vtable be modified or accessed at runtime? The answer is generally "no." Memory manipulation can provide access to the vtable, but determining the function signature for proper invocation remains challenging.

Vtables and Object Instances

Do all objects, regardless of whether they have virtual functions, have a vtable? The specification does not mandate vtables, so implementation details determine this behavior. Modern compilers typically only create vtables for classes with at least one virtual function.

Abstract Classes and Pure Virtual Functions

Abstract classes, with pure virtual functions, are handled differently. The language specification does not specify how pure virtual functions are represented in the vtable, leaving it up to the implementation.

One approach is to allocate a slot in the vtable but not assign an address to it, resulting in an incomplete vtable that requires derived classes to implement the function and complete the vtable.

Impact of Virtual Functions on Performance

Does the presence of virtual functions impact the performance of the entire class? The overhead is mainly associated with calling virtual functions, not with defining them. The time hit is only incurred when the virtual function is actually called. Space overhead is an additional concern, as each class requires a vtable.

Overriding Virtual Functions and Speed

Overriding a virtual function does not necessarily improve its execution speed. The derived class may create its own vtable, leading to additional space overhead.

Further Resources

For further exploration, consider the following resources:

• http://www.codersource.net/published/view/325/virtual_functions_in.aspx (via Wayback Machine)
• http://en.wikipedia.org/wiki/Virtual_table
• http://www.codesourcery.com/public/cxx-abi/abi.html#vtable

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