What's the Difference Between '=default' and '{}' for Default Constructors and Destructors in C ?

Understanding the Difference between "=default" and "{}" for Default Constructor and Destructor

While "=default" and "{}" might appear interchangeable for default constructors and destructors, there are key distinctions when considering non-virtual destructors and constructors.

Non-Virtual Destructors

When dealing with non-virtual destructors, the "=default" syntax plays a significant role. It instructs the compiler to automatically generate the destructor as it would for trivial classes, making the type considered trivial. On the other hand, "{}" creates a user-provided destructor, altering the triviality status of the class.

Non-Virtual Constructors

Similarly, for non-virtual constructors, using "=default" allows the compiler to generate a default constructor, maintaining triviality. However, "{}" specifies a user-provided constructor, potentially impacting the class's triviality.

Trivial Classes

In C 11, a trivial class is one that doesn't have any user-provided special member functions (default constructor, copy/move constructors/assignment, destructors). These functions are automatically generated, allowing optimizations like memcpy to be applied.

Example

Consider the following code:

struct Trivial
{
  int foo;
};

struct NotTrivial
{
  int foo;

  NotTrivial() {}
};

struct Trivial2
{
  int foo;

  Trivial2() = default;
};

• Trivial is trivial because it doesn't have any user-provided special member functions.
• NotTrivial is not trivial because it has a user-provided default constructor.
• Trivial2 is trivial even though it has a constructor marked as "=default," as this still invokes the compiler's default constructor generation.

Conclusion

While "=default" and "{}" may seem similar, they can have substantial implications for the triviality of classes when used with non-virtual destructors or constructors. "=default" retains the compiler's default behavior and preserves triviality, while "{}" creates user-provided functions and potentially alters the class's triviality status.

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