C++ data members: which category is shared by all instances of the class?

Introduction / Context:
Class design often distinguishes between per-object state and per-class state. When a value should be common across all objects (for example, a counter of instances or a configuration flag), C++ provides a specific storage class and access pattern to model this: static data members. Understanding this distinction avoids redundant storage and keeps invariants centralized.

Given Data / Assumptions:

• Per-object members exist separately in each instance.
• Per-class members exist once regardless of the number of instances.
• Access control (public/private/protected) is independent from whether a member is static or non-static.

Concept / Approach:

A static data member is allocated once and shared among all instances of the class. It can be accessed via the class name (e.g., T::count) or through an object (though the former is clearer). Public/private affects visibility, not multiplicity. “Inherited” is not a storage class; it merely describes where names come from in hierarchies. “Friend” is an access grant, not a data member category. Hence, the only option that encodes “shared by all instances” is static.

Step-by-Step Solution:

Verification / Alternative check:

Instantiate several objects and modify a static member through one of them; observe the updated value through all others and via the class name, confirming shared storage.

Why Other Options Are Wrong:

Public — concerns visibility, not sharing.

Inherited — not a storage specifier.

Friend — grants access to another function/class; not a member kind.

Common Pitfalls:

• Forgetting to define non-inline static data members in a translation unit (before C++17).
• Confusing static data members with static local variables inside functions.

Final Answer:

Static