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

Why C Requires User-Provided Constructor for Default Initialization of Const Objects

The C standard dictates that when default-initializing an object of a const-qualified type, the type must have a user-provided default constructor. This requirement arises due to the interplay between the POD (Plain Old Data) concept and object initialization in C .

In C , objects of non-POD classes are initialized by default. However, PODs, which are lightweight data types without constructors, validators, or destructors, are not initialized by default. This is because PODs are designed for performance-sensitive scenarios where explicit initialization is more efficient.

When initializing an object of a const-qualified type, the standard mandates a user-provided constructor to ensure that the object can be initialized. This is necessary because if the underlying class is a POD without a constructor, the object would remain uninitialized, rendering it useless.

Consider the example code:

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

struct A {
  A() {} // Why is this required?
  B b; // Just for illustration
};

int main() {
  const A a;
}

In this code, the object a is const-qualified. If the A class did not have a user-provided constructor, it would be considered a POD. Since PODs are not initialized by default, a would be uninitialized, making the code invalid.

However, the A class does have a user-provided constructor, which makes it non-POD. Therefore, a is default-initialized by calling the constructor, and the code compiles successfully.

The standard's requirement for a user-provided constructor is thus to ensure that objects of const-qualified types can be properly initialized and used.

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