What is the \'default\' Keyword and How Does It Affect Class Function Declarations in C ?

Understanding the "default" Keyword in Class Function Declarations

In modern C , the "default" keyword has become a valuable addition to class declarations. It plays a crucial role in specifying the compiler's behavior when it comes to generating certain functions.

Background:

When a constructor, destructor, or assignment operator declaration in a class does not provide a function body, the compiler typically provides a default implementation based on the class's design. However, using the "default" keyword explicitly instructs the compiler to use its generated version of the function.

Functionality of "default":

When "default" is used after a function declaration, it indicates that the compiler should generate the default implementation of that function. This means:

• The function body is not specified by the programmer.
• The compiler determines the function's behavior based on the class definition.
• The compiler-generated version is often optimized for efficiency and memory usage.

Example:

Consider the following C code snippet:

<code class="cpp">class C {
  C(const C&) = default;
  C(C&&) = default;
  C& operator=(const C&) & = default;
  C& operator=(C&&) & = default;
  virtual ~C() { }
};</code>

In this example, the declarations of the copy constructor, move constructor, copy assignment operator, and move assignment operator all use the "default" keyword. This instructs the compiler to generate these functions based on the class's design.

Benefits of Using "default":

• Simplifies code: The programmer does not need to manually define the default behavior of a function.
• Reduces errors: The compiler-generated functions are highly optimized and less prone to human error.
• Aligns with class design: The function behavior matches the intended semantics of the class.

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