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

Move Operation Generation in C

In C 98, copy constructors and assignment operators were automatically generated for classes without custom definitions. However, with the introduction of move semantics in C 11, the generation of move operations requires further consideration.

Automatic Generation of Move Operations

Move constructors and move assignment operators are automatically generated only if certain conditions are met:

• Default move semantics: The compiler generates move operations that perform a bitwise move of the object's data members.
• No user-defined destructors: If a class has a user-defined destructor, the compiler does not automatically generate move operations.
• No user-defined copy operations: Similarly, if a class has user-defined copy operations, the compiler does not automatically generate move operations.

Exceptions to Automatic Generation

In some cases, move operations are not automatically generated, even if the above conditions are met. For example:

• Class members with move-only types: If a class contains members with move-only types (i.e., types that cannot be copied), the compiler cannot automatically generate move operations.
• Trivial classes: Classes with only trivial data members (e.g., integers, strings) have implicitly defined copy and move operations, so the compiler does not generate custom implementations.

Additional Information

Howard Hinnant's presentation from the ACCU 2014 conference provides a comprehensive table summarizing the rules for automatic generation of special members, including move operations. The slides highlight that deprecated behavior is indicated by red squares.

To ensure move semantics are handled correctly, it is recommended to follow the "rule of 3" from C 98/03. This means explicitly declaring both copy members if the destructor is declared, or declaring at least one of the copy members.

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