When Are Move Constructors and Move Assignment Operators Automatically Generated in C 11?

Automatic Generation of Move Operations

In C 11, the introduction of move semantics raised questions about the circumstances under which move constructors and move assignment operators are automatically generated.

Under C 11, move operations are not automatically generated like copy constructors and copy assignment operators. The compiler only generates move semantics if certain conditions are met:

• No user-defined copy constructor or copy assignment operator exists. If either of these members is explicitly defined, no move operations are generated.
• No user-defined destructor is defined. Defining a destructor disables the generation of move semantics.
• The class has no non-copyable members. Move operations are not generated if the class contains members that cannot be moved, such as pointers to dynamically allocated memory.

Understanding these rules helps guide the implementation of classes that utilize move semantics. The table from Howard Hinnant's presentation provides a concise overview of the automatic generation of special members, including move operations:

Declaration	Copy Constructor	Copy Assignment Operator	Move Constructor & Move Assignment Operator
None	Generated	Generated	Not Generated
Copy Constructor	Not Generated	Generated	Not Generated
Copy Assignment Operator	Not Generated	Not Generated	Not Generated
Move Constructor	Not Generated	Not Generated	Not Generated
Move Assignment Operator	Not Generated	Not Generated	Not Generated
Destructor	Not Generated	Not Generated	Not Generated
Copy Constructor & Copy Assignment Operator	Not Generated	Not Generated	Not Generated
Copy Constructor & Move Constructor	Not Generated	Not Generated	Not Generated
Copy Constructor & Move Assignment Operator	Not Generated	Not Generated	Not Generated
Copy Assignment Operator & Move Constructor	Not Generated	Not Generated	Not Generated
Copy Assignment Operator & Move Assignment Operator	Not Generated	Not Generated	Not Generated
Move Constructor & Destructor	Not Generated	Not Generated	Not Generated
Move Assignment Operator & Destructor	Not Generated	Not Generated	Not Generated
Copy Constructor & Copy Assignment Operator & Move Constructor	Not Generated	Not Generated	Not Generated
Copy Constructor & Copy Assignment Operator & Move Assignment Operator	Not Generated	Not Generated	Not Generated
Copy Constructor & Move Constructor & Move Assignment Operator	Not Generated	Not Generated	Generated
Copy Assignment Operator & Move Constructor & Move Assignment Operator	Not Generated	Not Generated	Generated

By recognizing these rules, developers can ensure that their classes properly utilize move semantics and achieve optimal efficiency.

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