Why Must C Classes Have a User-Defined Default Constructor to Default-Construct a `const` Object?

Why Does C Require a User-Provided Default Constructor to Default-Construct a const Object?

The C standard mandates that if a program calls for the default initialization of an object of a const-qualified type, the underlying class type must possess a user-provided default constructor. This requirement stems from the fact that:

• POD (Plain Old Data) classes lack default initialization: If a POD class is not equipped with a user-defined constructor, it is eligible to be default-initialized. However, POD objects that lack initialization serve no practical purpose, as their state remains indeterminate.
• Const objects demand immediate initialization: Unlike POD objects, const objects require immediate initialization because their state cannot be altered once established. If a const object of a POD type is left uninitialized, it becomes useless as its value cannot be modified.

To prevent this quandary, the standard dictates that const objects cannot be constructed from POD types that lack a user-provided default constructor. This ensures that const objects are always initialized upon creation.

Distinction Between POD and Non-POD Classes

To clarify, the standard's requirement applies specifically to const objects of non-POD classes. POD classes, which lack virtual functions or base classes and have data members that are themselves PODs, can be default-initialized even without a user-defined constructor. In contrast, non-POD classes require a user-provided default constructor to enable default initialization.

Illustrative Example

Consider the following code snippet:

struct B {
  B(): x(42) {}
  int doSomeStuff() const { return x; }
  int x;
};

struct A {
  A() {} // Required by the standard
  B b; // Illustration purpose
};

int main() {
  const A a; // Valid due to user-provided default constructor in A.
}

In this example, the const object 'a' of struct A can be default-constructed because A has a user-provided default constructor, which satisfies the standard's requirement for non-POD classes.

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