Dynamic Method Dispatch in Compilers: Correct Choice and Why
Dynamic method dispatch is primarily used to implement runtime polymorphism—the ability for a program to choose which method implementation to invoke at runtime based on the actual (dynamic) type of an object, even when the code is written against a static type. This is the core purpose of mechanisms like virtual functions / virtual method tables (vtables) and related lookup strategies in compiled languages that support polymorphism.
In contrast:
- Lexical analysis is about tokenization and does not involve method overriding/late binding.
- Constant folding is an optimization performed by the compiler on expressions known at compile time; it does not require runtime type-based method selection.
- Register allocation is a backend compilation step that maps program variables to CPU registers to reduce memory traffic; it does not implement polymorphic behavior.
Therefore, the correct option is: (iii) Runtime polymorphism.
Key terms: Dynamic method dispatch; Runtime polymorphism; Virtual dispatch; Lexical analysis; Register allocation.
Runtime Polymorphism and Dynamic Dispatch (Virtual Functions)
The exam-style question asks which compiler/implementation task dynamic method dispatch is used for. Conceptually, it aligns with the language feature known as Runtime polymorphism: when you call an overridden method through a base-class reference, the program must determine the correct overridden implementation after object creation.
This requirement drives the need for Dynamic method dispatch mechanisms such as:
- Vtable for virtual method calls
- Late binding for overridden methods
Lexical analysis operates before semantic analysis and does not need per-object method lookup; it’s concerned with recognizing tokens like identifiers, keywords, and literals.
Constant folding is an optimization on expressions; it reduces work by computing compile-time-known values early, again unrelated to late-bound method selection.
Register allocation is a low-level mapping problem solved by the compiler backend; dynamic dispatch affects control flow and call targets, but the task of choosing registers is separate.
How dynamic method dispatch implements runtime polymorphism
- 1Step 1
The code is compiled against a static type (e.g., base class), so the compiler cannot directly bind the overridden method body.
- 2Step 2
At runtime, the program allocates an instance whose dynamic type determines which override should run.
- 3Step 3
The call uses a dispatch mechanism (e.g., vtable lookup) to find the method implementation corresponding to the object’s dynamic type.
- 4Step 4
Control transfers to the resolved implementation, completing the late binding behavior needed for runtime polymorphism.
Why the correct choice is Runtime Polymorphism
Match dynamic method dispatch to the compiler/language concept it enables
Common confusions (and how to avoid them)
Exam strategy
When you see “dynamic method dispatch,” think “virtual/late-bound calls” → the invoked method depends on the object’s runtime type.
Avoid mixing compiler phases
Lexical analysis and register allocation are compiler stages in different parts of the pipeline. Dynamic method dispatch is mainly a language semantics/runtime mechanism for late-bound calls.
Knowledge Check
Dynamic method dispatch is primarily used to implement: