Composition vs. inheritance: Is it illegal in C++ to declare objects of one class as data members of another class? Choose the most accurate statement.

Introduction / Context: This tests understanding of composition, a fundamental OO design technique where a class contains objects of other classes as members. C++ fully supports composition and relies on it for RAII and aggregation patterns. Declaring member objects is not only legal; it is idiomatic.

Given Data / Assumptions:

• Two user-defined class types exist.
• One class defines an object of the other class as a data member.
• Constructors and destructors follow normal C++ rules.

Concept / Approach: Composition embeds an object directly inside another object’s storage. Construction/destruction of member subobjects are automatically sequenced: members are constructed before the containing object’s body runs and destroyed in reverse order. Whether the member has a default constructor matters only for default-initialization; if it lacks one, you must initialize it explicitly in the owner’s member-initializer list. None of this makes composition illegal.

Step-by-Step Solution:

Verification / Alternative check: Try implementing a class that owns a std::string or std::vector. These are objects from the standard library; their presence as members exemplifies composition working seamlessly.

Why Other Options Are Wrong:

• “Only fundamental types allowed”: incorrect; user-defined types are normal members.
• “Only if default constructor exists”: you can initialize with parameters instead.
• “Must be pointer”: pointers are optional; direct members are common and safer.
• “Illegal if unrelated”: relationship is irrelevant; composition does not require inheritance.

Common Pitfalls: Forgetting to initialize non-default-constructible members in the initializer list; assuming composition implies ownership semantics identical to pointers (RAII usually makes it safer).

Final Answer: Incorrect — it is legal and common (composition).