In C++ class design, what is a default constructor and under what circumstances is this constructor automatically called by the language runtime?

Introduction / Context:
Default constructors are fundamental in C++ class design. They allow objects to be created without passing explicit arguments and are important for containers, arrays, and many library facilities that require default constructible types. Interviewers often start with the basic definition of a default constructor before moving on to more complex questions about constructor overloading and initialisation rules.

Given Data / Assumptions:

• We are dealing with C++ classes that can have one or more constructors.
• A constructor is a special member function with the same name as the class and no return type.
• A default constructor is defined either explicitly by the programmer or implicitly generated by the compiler.
• The question asks when and how a default constructor is used.

Concept / Approach:
The key idea is that a default constructor is a constructor that can be invoked without any arguments. It may have no parameters or may have all parameters with default values. The C++ runtime uses the default constructor when declaring objects without an explicit initialiser, when creating elements in some standard containers, and when zero initialising arrays of class types. If no constructors are declared, the compiler may generate a default constructor automatically, subject to certain conditions. The correct option must communicate both the argument free nature and the role in default initialisation.

Step-by-Step Solution:
Step 1: Recall that a typical default constructor looks like ClassName(); or ClassName(int x = 0); which allows calling it without arguments. Step 2: Recognise that when you declare ClassName obj; without parentheses, the compiler calls the default constructor to create obj. Step 3: Examine option (a), which states that a default constructor can be called with no arguments and is used when no explicit initialiser is provided. Step 4: Confirm that this matches the textbook definition for C++ default constructors. Step 5: Reject options that confuse default constructors with copy constructors or claim that they are never generated by the compiler.

Verification / Alternative check:
To verify, think about using a standard library container such as std::vector. When you write std::vector<MyClass> v(10);, the vector creates ten elements of type MyClass using the default constructor. If MyClass does not have a default constructor, this code will not compile. This example confirms the practical importance and the behaviour described in option (a).

Why Other Options Are Wrong:
Option (b) incorrectly describes the pattern of a copy constructor rather than a default constructor. Option (c) is wrong because default constructors are used when creating new objects, not when copying existing ones. Option (d) is incorrect because the compiler can and often does generate a default constructor automatically when no user defined constructors exist and when it is not inhibited by other language rules.

Common Pitfalls:
A common pitfall is to declare another constructor with parameters and then forget that this suppresses the compiler generated default constructor. In that case, attempts to default construct the class will fail to compile. Another mistake is to confuse zero initialisation with default construction; although related, these concepts are not identical. Knowing the definition of a default constructor and when it is automatically invoked helps avoid many subtle bugs in C++ programs.

Final Answer:
A default constructor is a constructor that can be called with no arguments and is used to create and initialise an object when no explicit initialiser is provided.