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When should you use a class vs a struct in C++?

In C++, both class and struct can define user-defined types, but they differ in default access and in conventional usage patterns. By understanding these distinctions and following good design practices, you can write clearer, more maintainable code.

1. Default Access Specifiers

  • class
    Members and base classes are private by default.

  • struct
    Members and base classes are public by default.

This difference in default access often influences how developers choose between class and struct in practice.

2. Conventional Usage

When to Use class

  1. Encapsulation & Data Hiding

    • If your design needs to encapsulate internal data and expose only a minimal public interface, a class is ideal.
    • Example: A complex data structure (like a balanced tree) where you want fine-grained control over what’s publicly accessible.

  2. Object-Oriented Programming

    • Classes are the go-to for inheritance, virtual functions, and other OOP features.
    • Typically, classes model behavior and responsibilities (methods, interfaces, etc.).

  3. Complex Logic

    • When you have significant logic (methods, invariants, private helper functions), a class emphasizes that there are “internals” you usually don’t expose directly.

When to Use struct

  1. Simple, Passive Data Holders

    • A struct is often used for Plain Old Data (POD) or near-POD types, meaning they primarily store data with minimal or no logic.
    • Example: A coordinate with x, y fields that’s just a simple aggregate of values.

  2. Public Data

    • If you plan for all members to be public and the object is more of a “dumb data container,” struct usage makes it obvious that these fields can be accessed directly without getters/setters.

  3. Small Utility Types

    • For quick groupings of related fields (like a small POD that you might pass around in a function), structs can reduce boilerplate by removing the need for explicit public: sections.

3. Design Considerations

  1. Clarity & Intent

    • If your “object” is little more than a bundle of fields, use a struct—this communicates that you’re storing basic data.
    • If it’s an abstraction with private data and public methods, use a class—this communicates that there’s some behavior or invariants to protect.

  2. Code Style & Consistency

    • Organizations often have coding guidelines like:
      • “Use struct for simple data aggregates with no invariants.”
      • “Use class for everything else.”
    • Sticking to a consistent convention makes code more predictable and maintainable.

  3. Inheritance and Polymorphism

    • Although structs can also inherit and override virtual methods, using a class for polymorphic hierarchies is more standard and signals that the type has complex behavior.

  4. Access Modifiers

    • Even if you choose struct, you can still explicitly mark members as private: or protected: as needed.
    • Similarly, you can make parts of a class public if that suits your design better.

4. Performance and Language Semantics

  • Performance: There’s no performance difference solely based on using class vs struct. The compiler treats them similarly except for the default accessibility rules.
  • Language Semantics: Both can hold member functions, operators, constructors, destructors, etc. The distinction is more about intent and convention than functionality.

5. Real-World Examples

Example 1: Simple Data Aggregate

struct Point {
    double x;
    double y;
};
  • Used as a convenient way to group x and y. No behavior beyond storing data.

Example 2: Encapsulated Class

class BankAccount {
public:
    BankAccount(double initialBalance) : balance(initialBalance) {}
    
    void deposit(double amount) { balance += amount; }
    void withdraw(double amount);

    double getBalance() const { return balance; }

private:
    double balance;
};
  • Uses encapsulation: Data is private, and only specific operations (deposit/withdraw) are allowed.

Why This Matters in Coding Interviews

Interviewers often check whether candidates:

  • Understand C++ encapsulation and access control.
  • Can design types that reflect their intended use.
  • Know best practices for simple aggregates vs. classes with behavior.

Showing you can articulate the difference between classes and structs—and choose appropriately—demonstrates your fluency with modern C++ design principles.

Further Learning Resources

To deepen your knowledge of C++ and get ready for coding interviews, check out DesignGurus.io:

  1. Grokking the Coding Interview: Patterns for Coding Questions

    • Master key problem-solving patterns and algorithms frequently tested at top tech companies.

  2. Grokking Data Structures & Algorithms for Coding Interviews

    • Gain proficiency in performance-critical data handling—a must for complex software roles.

You can also explore their YouTube channel for free expert insights on system design and coding. If you prefer personalized feedback, consider booking a Coding Mock Interview or joining their Interview Bootcamp.

Key Takeaway

  • Use struct for simple, passive data holders where members can be publicly accessed.
  • Use class for encapsulated types or where you expect polymorphic behavior (inheritance, virtual functions) and want to control access by default.
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