In C++, what is the key difference between a class and a structure, especially in terms of default member access and typical usage?

Introduction / Context:
The distinction between class and structure in C++ is a classic interview question. Many programmers know that both can hold data and functions, but they may not clearly remember the default access rules and typical design intent. This question tests basic C++ object oriented programming understanding and helps clarify common confusion inherited from C language structures.

Given Data / Assumptions:

• We are working in standard C++, not C language.
• Both class and struct keywords can declare user defined types.
• We care about syntax level differences and common usage conventions.

Concept / Approach:
In C++, structures evolved from C but gained almost all capabilities of classes. They can have member functions, constructors, destructors, and even inheritance and polymorphism. The key syntactic difference is the default access specifier: members of a class are private by default, whereas members of a struct are public by default. Beyond that, usage is mostly a convention: developers tend to use struct for plain data aggregates and class for full featured objects with encapsulation.

Step-by-Step Solution:
Step 1: Recall that both class and struct can contain data members and member functions in C++. Step 2: Check the default access rules: class uses private as the default for members and base classes. Step 3: Remember that struct uses public as the default for its members and base classes. Step 4: Understand that you can explicitly specify public, protected, or private in both, overriding the default. Step 5: Conclude that the principal language defined difference is the default access level and that conventional usage follows from this.

Verification / Alternative check:
A simple code example demonstrates this: declaring a class with an int member and attempting to access it directly from main will fail unless you mark it public. Doing the same with a struct will compile because the member is public by default. If you explicitly set access specifiers, the behaviour is identical, confirming that the only built in difference is the default access level.

Why Other Options Are Wrong:
Option B is wrong because C++ structures can absolutely contain member functions, including constructors and operators. Option C is incorrect since both classes and structures can participate in inheritance and polymorphism. Option D is wrong because it denies the real difference in default access rules, which is precisely what the language specifies.

Common Pitfalls:
Many learners mistakenly think that structures are limited like in C and cannot have methods, or they forget the default access difference and unintentionally expose members. A good practice is to choose struct for simple passive data objects with public fields and class for more complex, encapsulated types, while always being explicit about access specifiers for clarity.

Final Answer:
The correct explanation is that in C++, the only formal difference is that class members are private by default, while structure members are public by default; classes are usually used for OOP, and structures for simple data groups.