What are the Advantages of Using \'= default\' for Default Constructors in C 11?

The Significance of the "Default" Syntax in C 11

In C 11, the new syntax "= default" provides a concise and explicit way to define and handle constructors, destructors, and copy/move assignments.

Consider the example provided:

struct S { 
    int a; 
    S(int aa) : a(aa) {} 
    S() = default; 
};

This code defines a struct S with a constructor that takes an integer parameter and an empty default constructor. The "= default" syntax here signals the compiler to generate a default constructor with an empty body.

Why Not Simply "S() {}"?

One may wonder why "= default" is used instead of simply "S() {}." While both constructors will behave similarly, the "S() = default;" syntax has several advantages:

• Aggregate and Trivial Types: Using "= default" ensures that the class remains an aggregate and trivial type. This is crucial for classes that need these properties, such as POD (Plain Old Data) types.
• Uniformity: The "= default" syntax provides consistency in handling special member functions. It can be used for copy/move constructors and destructors besides default constructors.
• Explicit Intent: Using "= default" explicitly states the intention to create a default constructor with a specific behavior. This improves code readability and reduces the likelihood of errors.

Ensuring Correctness

The defaulted default constructor is designed to behave exactly like a user-defined default constructor with no initialization list and an empty compound statement. However, if a class contains non-trivial members (e.g., non-default constructible members), a user-provided default constructor would be mandatory, and "= default" would not be appropriate.

In addition to generating the constructor, "= default" also ensures that the correct exception specification and constexpr properties are set. This ensures that the class behaves as expected and aligns with the implicit constructor that would have been generated without "= default."

In conclusion, the "= default" syntax in C 11 provides a succinct and explicit way to define and handle special member functions, enhancing code readability, ensuring correctness, and maintaining compatibility with older C versions. By using this syntax, programmers can simplify their codebase and ensure predictable behavior across different compilers and platforms.

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