🧱 When Should You Call Base Class Constructors in C++?

Disclaimer: ChatGPT generated document.

In C++, inheritance allows us to build hierarchies of classes where a derived class inherits from one or more base classes. But when exactly should you explicitly call the base class constructor in your derived class?

If you've ever written code like this and wondered whether it’s necessary:

class MyException : public std::nested_exception {
public:
    MyException() : std::nested_exception() {}  // ❓Is this required?
};

This article clears the fog with simple rules, concrete examples, and best practices.

🧠 Rule of Thumb

Case	Do You Need to Call Base Constructor?
Base has a non-default constructor	✅ Yes
Base has a default constructor (no-args)	❌ No
You want to pass arguments to the base	✅ Yes
Base is a standard tag (e.g. `std::nested_exception`)	❌ No
You're dealing with virtual inheritance	⚠️ Yes, but only in the most-derived class

✅ When You Must Call the Base Constructor

1. No Default Constructor

If the base class does not provide a no-argument constructor, you must initialize it explicitly:

class Base {
public:
    Base(int x);
};

class Derived : public Base {
public:
    Derived() : Base(42) {}  // ✅ Required
};

2. You Need to Pass Custom Arguments

Even if a default constructor exists, you can override it by calling another one:

class Base {
public:
    Base(std::string msg);
};

class Derived : public Base {
public:
    Derived() : Base("Custom message") {}  // ✅ Customize base behavior
};

❌ When You Don’t Need to Call the Base Constructor

1. Default Constructor Exists

If the base class has a no-argument constructor and you're happy with its default behavior, C++ does it for you:

class Base {
public:
    Base() {}
};

class Derived : public Base {
public:
    Derived() {}  // ✅ No need for : Base()
};

2. Base is a Marker or Utility Type

Types like std::nested_exception, std::exception, and std::ios_base often have default constructors. There's no need to explicitly call them unless you want to pass arguments:

class MyException : public std::nested_exception {
public:
    MyException() {}  // ✅ std::nested_exception() is called automatically
};

This makes your code cleaner and idiomatic.

⚠️ Special Case: Virtual Inheritance

If a class inherits virtually from a base class, only the most-derived class should initialize that base:

class A {
public:
    A(int) {}
};

class B : virtual public A {};
class C : public B {
public:
    C() : A(5) {}  // ✅ Must initialize A here, not in B
};

Failing to do so results in a compiler error.

🧰 Bonus: What Happens If You Over-Initialize?

Calling a default constructor unnecessarily:

Derived() : Base() {}

…is not wrong, but it's redundant, potentially misleading, and unnecessary clutter. Keep your code clean by letting the compiler do its job.

✅ Summary Table

Base Class Condition	Call Base Constructor?
No default constructor	✅ Required
Passing arguments	✅ Required
Default constructor is sufficient	❌ Skip it
Using `std::nested_exception`	❌ Skip it
Virtual base — in most-derived class	✅ Required
You’re not sure	❓ Prefer clarity — call if needed only

✍️ Final Thoughts

Being explicit where necessary and implicit where idiomatic is part of writing clean, modern C++. Understanding when base constructors are called lets you avoid redundant code and prevents bugs in class hierarchies.

So next time you write:

class Derived : public std::runtime_error, public std::nested_exception {
public:
    Derived(const std::string& msg)
        : std::runtime_error(msg), std::nested_exception()  // 🚫 unnecessary
    {}
};

You’ll know: that last constructor call can — and should — go!

MangaD/cpp_base_class_constructor.md