In C++ object oriented design, how many primary techniques are commonly discussed for reusing existing classes in a class hierarchy, such as inheritance and composition or aggregation?

Introduction / Context:
Code reuse is a central goal of object oriented programming in C++. When discussing class hierarchies and design, two primary techniques are usually highlighted: inheritance (reusing an interface and implementation by deriving from a base class) and composition or aggregation (reusing a class by holding it as a member). This question focuses on recognizing these two main reuse mechanisms rather than counting every possible variation.

Given Data / Assumptions:

• We are focusing on high level design techniques for reusing classes.
• We consider inheritance and composition or aggregation as distinct approaches.
• We are not counting minor subcategories such as single vs multiple inheritance separately.

Concept / Approach:
Inheritance allows a derived class to reuse and extend the behaviour of a base class. Composition (and the closely related aggregation) allow a class to reuse another by storing it as a member object and delegating work to it. Most C++ design discussions highlight these as the two primary reuse mechanisms. Other details such as multiple inheritance or templates are refinements or orthogonal concepts. Therefore, it is reasonable in this interview style question to answer that there are two main ways of reuse in a class hierarchy context.

Step-by-Step Solution:
Step 1: Identify inheritance as the first technique: class Derived : public Base { /* reuse Base */ }; Step 2: Recognize composition or aggregation as the second: class A { B member; } where A reuses class B by containing it. Step 3: Notice that both techniques let you build new classes from existing ones but with different coupling and flexibility characteristics. Step 4: Count these as two primary, conceptually distinct forms of reuse. Step 5: Choose 2 as the correct answer in the context of a simple interview question.

Verification / Alternative check:
Textbooks and design guidelines for C++ often discuss "inheritance versus composition" as a key design choice. Many even recommend "prefer composition over inheritance" when possible. This binary comparison reinforces the idea that these are the two main categories of reuse, supporting the choice of 2 as the number of primary ways of reusing classes in a hierarchy or design.

Why Other Options Are Wrong:
Option A (1) would ignore either inheritance or composition, missing a fundamental technique. Options C and D count more techniques than are typically grouped as primary reuse mechanisms in introductory design discussions, unless one splits inheritance into multiple subtypes, which is not the usual intent of this question.

Common Pitfalls:
A common pitfall is to overuse inheritance when composition would provide a cleaner and more flexible design. Another mistake is to confuse reuse with simple copy and paste duplication, which increases maintenance cost instead of reducing it. Understanding that inheritance and composition are the two key structured reuse strategies helps developers evaluate tradeoffs and design more robust C++ class hierarchies.

Final Answer:
There are 2 primary techniques commonly discussed for reusing classes in C++ object oriented design: inheritance and composition or aggregation.