When Should I Use '=default' vs '{}' for Default Constructors and Destructors in C ?

Understanding the Distinction Between "=default" and "{}" for Default Constructors and Destructors

The question of whether "=default" behaves differently from an empty definition ("{}") for default constructors and destructors has nuanced implications. While they may appear similar for virtual destructors, their significance diverges when considering constructors and non-virtual destructors.

Default Constructors

Unlike with destructors, the distinction between "=default" and "{}" for default constructors has a significant impact.

Using "{}" creates a user-provided constructor, altering the semantics of the class. It places the responsibility of memory management within the constructor's logic, rendering the class non-trivial according to C 11 criteria. This prevents the compiler from engaging in specific optimizations regarding memory allocation.

=default, on the other hand, delegates the creation of the default constructor to the compiler, ensuring the class remains trivial. By indicating that a user-provided default constructor is not intended, it triggers optimizations related to memory handling.

Destructors

For virtual destructors, "=default" and "{}" have comparable effects, as both enable virtual destruction. However, if the destructor is non-virtual, these two options have contrasting implications.

An empty definition ("{}") for a non-virtual destructor yields a user-provided destructor, potentially altering destruction semantics. As a result, the class is no longer considered trivial.

By contrast, =default for a non-virtual destructor instructs the compiler to create a default destructor automatically. This preserves the trivial nature of the class by indicating that no user-provided destructor is necessary.

Conclusion

The choice between "=default" and "{}" for constructors and destructors depends on the desired behavior and coding constraints. For virtual destructors, both options are functionally equivalent. For non-virtual destructors, "=default" retains the class's trivial nature, while "{}" introduces user-defined destruction logic.

In the context of default constructors, "=default" ensures triviality and compiler-generated initialization, while "{}" enables custom memory management and non-triviality. These distinctions impact performance optimizations and memory allocation semantics, necessitating careful consideration based on the desired object behavior.

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