Writing a well-structured C++ class isn’t easy. While object-oriented programming (OOP) is often summarized by abstraction and encapsulation, actually applying these concepts in C++ can be tricky.
This post explores how to design a “qualified” class in C++, balancing clarity, maintainability, and modern language features.
1. Rethinking Object-Oriented Programming #
OOP is a programming philosophy, not just a toolbox of tricks. Features like inheritance, polymorphism, and overloading are tools—not the essence.
Good C++ code should prioritize clarity and efficiency over unnecessary complexity. Remember: you’re writing for humans first, not for showing off language features.
2. Organizing Class Source Files #
Traditionally, C++ classes are split into .h (declaration) and .cpp (implementation):
// complex.h
class Complex {
public:
Complex(double r, double i);
};
// complex.cpp
Complex::Complex(double r, double i) {
// implementation
}
Modern practice often merges declaration and implementation into a single .hpp file:
// complex.hpp
class Complex {
public:
Complex(double r, double i) {
// implementation
}
};
This pattern is common in libraries like Boost and simplifies class management.
3. Design Principles for Classes #
3.1 Minimize Inheritance #
Prefer composition over inheritance unless it improves clarity. Inheritance introduces complexity (virtual functions, multiple inheritance, etc.) that may not be worth it.
3.2 Control Inheritance Depth #
Avoid deep hierarchies. More than three levels of inheritance often signals poor design.
3.3 Single Responsibility #
A class should ideally serve one purpose. If a class feels overloaded, reconsider your design or introduce abstractions.
4. Practical C++ Techniques #
4.1 Final Classes and Inheritance Rules #
class Complex final {
// cannot be inherited
};
class Base {};
class Derived final : public Base {
// public inheritance, explicitly final
};
Use final to enforce design choices at compile time.
4.2 The “Rule of Six” (C++11 and Beyond) #
A modern C++ class may define six special functions (nicknamed 3-2-1):
- Constructors (default, copy, move)
- Assignment operators (copy, move)
- Destructor
Let the compiler handle defaults when possible:
class Complex {
public:
Complex() = default;
~Complex() = default;
};
Disable operations explicitly with delete:
class Complex {
public:
Complex(const Complex&) = delete;
Complex& operator=(const Complex&) = delete;
};
4.3 Explicit Conversion #
class A {
public:
explicit A(int x) { ... }
explicit operator int() { ... }
};
Adding explicit prevents unwanted implicit conversions, forcing clarity.
4.4 Member Initialization #
class Demo final {
public:
Demo() = default;
Demo(int v) : x(v) {}
private:
int x = 0;
std::string s = "default";
};
Member initialization ensures consistent defaults and avoids uninitialized variables.
4.5 Type Aliases #
using uint_t = unsigned int;
class Demo final {
public:
using str_t = std::string;
using vec_t = std::vector<std::string>;
private:
str_t name = "demo";
vec_t items;
};
Aliases shorten verbose types and improve readability.
4.6 Delegating Constructors #
class Demo final {
public:
Demo(int v) : x(v) {}
Demo() : Demo(0) {} // delegates to the first constructor
private:
int x;
};
Prevents code duplication by chaining constructors.
5. Summary Checklist: Writing a Qualified C++ Class #
✅ Prefer composition over inheritance
✅ Keep inheritance shallow (≤ 3 levels)
✅ Ensure single responsibility per class
✅ Use final when a class shouldn’t be extended
✅ Follow the Rule of Six (or rely on compiler defaults)
✅ Mark single-argument constructors and conversion operators as explicit
✅ Initialize members directly with default values
✅ Use using for type aliases to improve readability
✅ Apply delegating constructors to reduce duplication
Final Thoughts #
C++ is notoriously complex, but writing a “qualified” class doesn’t mean mastering every corner of the language. It means adopting practical, modern habits that keep your classes clean, maintainable, and safe—while leveraging the power of C++ when it truly matters.