Do built-in types exhibit move semantics, and if so, how do they define after-move behavior?

Do Built-In Types Possess Move Semantics and Define After-Move Behavior?

The provided code snippet demonstrates the apparent modification of a built-in type (int) within a function that receives an r-value reference to it. However, the concept of "move semantics" raises questions about the actual behavior of built-in types in this context.

Understanding Built-In Types and Move Semantics

Built-in types, unlike user-defined types, do not possess resources or manage their own memory. Instead, they are the resources themselves. Moving an object implies transferring ownership of its resources, which is not applicable to built-in types since they hold no external resources.

Impact of std::move on Built-In Types

std::move transforms an lvalue into an xvalue, allowing it to bind to r-value references. It does not trigger any constructors or cause any runtime actions. At the type level, it merely modifies the value category.

Rvalue References and Modifications

Rvalue references remain references, connecting to the original object. When an r-value reference is passed to a function, the function can modify the original object through the reference. This is what happens in the provided example, where the function increments the original int.

No Move Constructor for Fundamental Types

Fundamental types, such as ints, lack move constructors. If a function receives a fundamental type by value, the move degrades to a copy. Consequently, the original object's value is copied into the function parameter, and any modifications made within the function do not affect the original object.

Well-Defined Behavior for Built-In Types After Move

The behavior observed in the example, where the original int is modified through an r-value reference, is well-defined by the C standard. It is the result of the lack of move constructors and the functionality of r-value references.

Conclusion

While built-in types do not possess move semantics in the traditional sense, std::move can be applied to them to create r-value references. These r-value references, when passed to functions, allow for modifications to the original objects. However, fundamental types do not have move constructors, so any "move" operations on them effectively become copies. The behavior of built-in types after move is well-defined and follows the principles of r-value references and the lack of move constructors for fundamental types.

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