Why Do Default-Constructed Const Objects in C Require a User-Defined Default Constructor?

Why is a User-Defined Default Constructor Required for Default-Constructed Const Objects in C ?

The C standard dictates in section 8.5 that for default-initialized const objects, their type must be a class with a user-provided default constructor. Understanding this requirement necessitates comprehending the distinction between POD (Plain Old Data) classes and non-POD classes.

POD vs. Non-POD Classes

POD classes are those without user-defined constructors, destructors, or virtual member functions. They can be initialized with uninitialized memory, allowing efficient memory allocation and direct memory manipulation.

Non-POD classes, on the other hand, are those that violate any of the POD constraints. They must be initialized before use, and attempts to initialize them with uninitialized memory result in undefined behavior.

Requirement for Default Constructors

The standard enforces the requirement of a user-provided default constructor for default-constructed const objects because:

• Ensuring Object Utility: If a POD class doesn't have a user-defined constructor, it remains uninitialized by default. Declaring a const object of an uninitialized POD class would render it useless, as its value cannot be modified.

  POD p1; // uninitialized - can be assigned later
  const POD p2; // uninitialized - error, cannot be modified

• Facilitating Non-POD Initialization: User-defined constructors make classes non-POD. Declaring a const object of a non-POD class guarantees its initialization, regardless of whether an initializer is provided.

  nonPOD_B b1; // initialized
  const nonPOD_B b2; // initialized

Therefore, by requiring a user-provided default constructor for default-constructed const objects, the C standard ensures that even const objects of POD classes are properly initialized and, thus, usable.

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