Class or Structure in C++: When to Choose What? Analysis of the Main Question

Class or Structure in C++: The Eternal Debate

Imagine a situation: you are writing code in C++, and you need to create a new data type. A question immediately pops into your head: “When should I use class, and when should I use struct?”. This question plagues not only beginners — on Stack Overflow it has gathered thousands of views and dozens of heated discussions. And it's no wonder: in C++, these two keywords, at first glance, do the same thing, but with nuances.

In this article, we will once and for all figure out what the real difference between class and struct is, when to choose what, and how not to get caught out. Let's go!

The Main Difference: Default Access Modifiers

If you think that class and struct are completely different entities, you are mistaken. In C++, the only fundamental difference between them is the default access modifier:

\n
• struct: all members (fields and methods) are public by default.
\n
• class: all members are private by default.
\n

That's it. There are no other differences at the language level. No performance differences, no magic. Only accessibility. Let's look at examples of how this works.

Example with struct

struct Point {\n    int x;\n    int y;\n};\n\n// Can access directly\nPoint p;\np.x = 10;\np.y = 20;

Example with class

class Point {\n    int x;  // private by default!\n    int y;\npublic:\n    void setX(int val) { x = val; }\n    void setY(int val) { y = val; }\n};\n\nPoint p;\n// p.x = 10; // Error! x is private\np.setX(10);   // This is allowed

As you can see, the difference is only in how you “hide” data by default. But this does not mean that choosing between class and struct is a matter of taste. There are established practices.

When to Choose struct?

In the C++ community, there is an unwritten consensus: struct is for simple data. Use struct when your type is simply a set of public fields without complex logic. Classic examples:

\n
• Point coordinates (x, y)
\n
• RGB color (r, g, b)
\n
• Configuration (width, height, title)
\n
• Operation result (success, errorMessage)
\n

In such cases, there is no need to hide data — it is public anyway. struct makes the code cleaner and shorter. Additionally, struct is excellent for POD-types (Plain Old Data) — structures that can be copied with a simple memcpy.

Example of Correct struct Usage

struct Vector2 {\n    float x;\n    float y;\n    \n    // Can add simple methods\n    float length() const {\n        return std::sqrt(x*x + y*y);\n    }\n};\n\n// Used as a simple data container\nVector2 v = {3.0f, 4.0f};\nstd::cout << v.length(); // 5.0

Note: even if a struct has methods, that's fine. The main thing is that the fields remain public, and the type does not require complex invariant logic.

When to Choose class?

Class is the default choice for full-fledged objects. Use class when:

\n
• You have complex logic that requires encapsulation.
\n
• You want strict control over how data is modified.
\n
• Your type has invariants (e.g., “age cannot be negative”).
\n
• You are using inheritance, virtual functions, polymorphism.
\n

A classic example is the “Bank Account” class. The balance should not be public, otherwise someone might accidentally set a negative value.

Example of Correct class Usage

class BankAccount {\nprivate:\n    double balance;\n    std::string owner;\n    \npublic:\n    BankAccount(const std::stri