Does C Implicitly Zero-Initialize Data Members in Its Default Constructor?

Does C Provide an Implicit Default Constructor?

Background:

The existence of an implicit default constructor in C has raised questions among developers. Textbook claims suggest that the compiler generates one when none is explicitly declared, theoretically initializing data members to zero.

Default Constructor Implementation

If no constructor is defined for a class, the compiler creates a default constructor. Its behavior is as follows:

• Constructs the base class (if one exists) with a default constructor.
• Constructs each member variable in order of declaration, also using default constructors.

For POD (Plain Old Data) types, no explicit constructor exists, but the default behavior equates to no action.

Additional Implications

• Copy Constructor, Copy Assignment Operator, Destructor: If none of these are defined, the compiler provides default implementations:

  • Copy Constructor: Copies base and member variables.
  • Copy Assignment Operator: Updates base and member variables.
  • Destructor: Destroys base and member variables in reverse order.

• Move Constructor, Move Assignment Operator: If none are defined, the compiler again provides defaults:

  • Move Constructor: Moves base and member variables.
  • Move Assignment Operator: Moves base and member variables, returns a reference to this object.

Conclusion:

While the compiler does provide an implicit default constructor, it does not zero-initialize data members. The constructor's behavior is to default-construct base classes and member variables. This behavior, along with the default implementations for copy/move constructors and assignment operators, ensures consistent object creation and manipulation in C .

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