Virtual Functions

Virtual Function Mechanics

1. Compile-time: The compiler emits metadata (e.g., a vtable) and arranges for a per-subobject pointer (often called a vptr) in polymorphic types.
2. Runtime: The vptr selects the appropriate function implementation (dynamic dispatch).

• Pointers to the class's virtual functions (possibly with thunks for adjustments)
• A way to reach RTTI (type_info), implementation-dependent
• Offsets/adjusters for multiple/virtual inheritance when needed

Pure Virtual Functions & Abstract Classes

• Make a class abstract (it cannot be instantiated).
• Must be overridden in a concrete derived class for that class to be instantiable.
• May still provide a definition in the base (rare but legal).

override & final Keywords

• Marks a function as overriding a virtual function in a base; the compiler diagnoses mismatches.

• Applied to a virtual function: prevents further overriding.
• Applied to a class: prevents further inheritance from that class (but methods in that class may still override its base).

Virtual Destructors

1. Declare base destructors virtual when deleting objects via base pointers/references.
2. Derived destructors are implicitly virtual if the base destructor is virtual.
3. Destruction order is derived first, then base.

Virtual Calls in Constructors/Destructors

• Overridden functions in derived classes are not called from base constructors/destructors.
• Calling a pure virtual function in a constructor/destructor leads to a runtime error ("pure virtual call").

Performance Optimization

• Devirtualization: Compilers may resolve some virtual calls at compile time (especially on final classes/methods or with whole-program optimization).
• Use final on classes/methods when further overriding is unnecessary.
• Profile-guided and link-time optimizations can improve indirect-call performance.
• Avoid virtual calls in tight, performance-critical loops; consider CRTP/templates for compile-time polymorphism when appropriate.