Why Does C Require a User-Defined Default Constructor for Default-Constructing Constant Objects?

Why C Insists on a User-Provided Default Constructor for Default-Constructing Const Objects

The C Standard stipulates that default-constructing a const object requires a user-provided default constructor. Understanding the rationale behind this requirement enhances our grasp of object initialization in C .

The Purpose of User-Provided Default Constructor for Const Objects

The primary reason for this requirement is to ensure the usefulness of the object when it's initialized via default construction. If the class lacks a user-defined constructor, it qualifies as a POD (plain old data) class. POD classes remain uninitialized by default. Thus, attempting to initialize a const object of an uninitialized POD class would yield an object of little practical value.

POD vs. Non-POD Classes

To illustrate this concept, consider the struct POD without a user-defined constructor:

struct POD
{
  int i;
};

POD p1; // uninitialized, but we can assign a value later
POD p2 = POD(); // initialized

const POD p3 = POD(); // initialized
const POD p4; // error - uninitialized and cannot be modified

As evident from the code, uninitialized POD classes can be problematic. However, if we add a user-defined constructor to the class, it becomes non-POD:

struct nonPOD_A
{
    nonPOD_A() {} // this makes non-POD
};

nonPOD_A a1; // initialized
const nonPOD_A a2; // initialized

Standard Excerpt and Interpretation

The C Standard states that "If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor." This can be interpreted as follows:

If we declare a const object of a non-POD class without providing an initializer, the default constructor is automatically invoked. However, if the class is POD, the object remains uninitialized. Hence, requiring a user-defined default constructor for const objects ensures their proper initialization and usefulness.

Conclusion

In conclusion, C demands a user-provided default constructor for default-constructing const objects to guarantee that the object remains initialized and useful upon its creation. Understanding the distinction between POD and non-POD classes is crucial in this context. By adhering to this rule, C ensures that const objects initialized via default construction serve their intended purpose effectively.

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