In C++, both classes and structs are user-defined data types that traditionally serve the purpose of aggregating data and behavior into a single entity. While they are fundamentally similar in how they are used, there are semantic differences and conventions that guide when to use one over the other. This knowledge is crucial for writing clear, effective, and maintainable C++ code.

Basic Differences: Syntax and Access Modifiers

The most immediate difference between a class and a struct in C++ lies in the default access level and inheritance type. For classes, the default access is private, while for structs, it is public. This difference influences how other parts of the code (or other types) interact with the data and functionality encapsulated within the class or struct.

1struct MyStruct {
2    int x; // public by default
3};
4
5class MyClass {
6    int y; // private by default
7};

Semantic Usage

1. Use of Struct:

A struct should be primarily used for passive structures with public access that hold data but do not require substantial methods or functions. They are best suited for data entities where manipulation of the data is transparent and straightforward, without the complexities of behavior that classes might encapsulate.

Example:

1struct Point {
2    double x;
3    double y;
4};
5
6void translate(Point &p, double dx, double dy) {
7    p.x += dx;
8    p.y += dy;
9}

2. Use of Class:

A class is ideal for active entities that encapsulate both data and behaviors, with greater emphasis on the complexities of these behaviors and the need for abstraction and encapsulation. Classes are suitable for when access control is necessary, or when leveraging features like inheritance and polymorphism.

Example:

1class Rectangle {
2    double width;
3    double height;
4
5public:
6    Rectangle(double w, double h) : width(w), height(h) {}
7
8    double area() const {
9        return width * height;
10    }
11
12    void resize(double factor) {
13        width *= factor;
14        height *= factor;
15    }
16};

Performance Considerations

Both structs and classes are treated the same way by the C++ compiler, so there is no inherent performance benefit to using one over the other. The choice between using a struct or a class should be based on design considerations rather than performance.

Inheritance and Polymorphism

Both structs and classes can use inheritance and polymorphism in C++. The key difference is the default access and inheritance types, as previously mentioned. Structures can be more transparent in cases of simple inheritance hierarchies used primarily for aggregating data.

Summary Table

Subtopics: Constructors and Member Functions

While both classes and structs can have constructors and other member functions, it is more common to see complex constructor logic and numerous member functions in classes due to the emphasis on behavior rather than just data.

1class Timer {
2private:
3    unsigned long start;
4public:
5    Timer() : start(now()) {}
6
7    void reset() {
8        start = now();
9    }
10
11    unsigned long elapsed() const {
12        return now() - start;
13    }
14};
15
16struct Date {
17    int day, month, year;
18};

Conclusion

Choosing between a class and a struct in C++ should be guided by the principles of encapsulation and intended usage: use structs for simple data entities and use classes for entities that require robust behavior and encapsulation. This approach not only keeps the code logical and organized but also takes advantage of the semantic strengths of each construct.