How does std::move() convert lvalues to rvalues in C ?

Extended Understanding of std::move's Conversion to Rvalues

In C , std::move() plays a crucial role in converting expressions to rvalues (right-value references). However, its implementation can be confusing to grasp. This article aims to provide a clear understanding of the move function and its behavior with both lvalues and rvalues.

std::move()'s Implementation

The std::move() function, as implemented in the MSVC standard library, takes an rvalue reference (&&) argument and returns an rvalue reference. This allows std::move() to handle both lvalues (left-value references) and rvalues effectively.

Rvalue Binding

When std::move() is called with an rvalue, such as a temporary object, the _Arg reference parameter binds directly to the rvalue. This is straightforward since an rvalue reference can bind to an rvalue.

Lvalue Binding

When std::move() is called with an lvalue, the _Arg reference parameter binds to a lvalue reference (Object&). This raises the question of how an rvalue reference can bind to an lvalue.

Reference Collapsing

To understand this behaviour, we need to consider C 11's rules for reference collapsing. These rules state that:

Object & & = Object &
Object & && = Object &
Object && & = Object &
Object && && = Object &&

According to these rules, Object& && is equivalent to Object&, which is a plain lvalue reference that can bind to lvalues.

Effect of remove_reference

std::move() uses std::remove_reference to remove any existing references from the type of the argument, resulting in a typename tr1::_Remove_reference::_Type. Consequently, it ensures that the returned value is always an rvalue reference, regardless of the type of the argument.

Advantages of the Implementation

The implementation of std::move() offers several advantages:

• It enables perfect forwarding by allowing both lvalues and rvalues to bind to the argument.
• It ensures that the returned value is always an rvalue reference, which is essential for move semantics in C .

Conclusion

std::move()'s implementation is intricately designed to convert expressions to rvalues effectively, handling both lvalues and rvalues with reference collapsing and ensuring a consistent rvalue reference return type. This mechanism plays a critical role in the use of move semantics in C .

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