What Does std::move() Do to lvalues?
std::move() is a powerful tool in C++ for facilitating perfect forwarding. Yet its implementation can raise a few eyebrows upon first glance. To clarify how std::move() achieves this, let's delve into the intricacies of its implementation.
Understanding the 'Move' Function
We'll start with the simplified version of the 'move' function template:
template <typename T>
typename remove_reference<T>::type&& move(T&& arg)
{ return static_cast<typename remove_reference<T>::type&&>(arg); }Case 1: Passing an Rvalue
When std::move() is called with an rvalue (a temporary object or an expression that evaluates to an rvalue), the move template is instantiated as follows:
// move with [T = Object]:
remove_reference<Object>::type&& move(Object&& arg)
{ return static_cast<remove_reference<Object>::type&&>(arg); }Since remove_reference
Object&& move(Object&& arg) { return static_cast<Object&&>(arg); }As anticipated, the function simply casts its rvalue argument to an rvalue reference.
Case 2: Passing an Lvalue
Now, the interesting case arises when std::move() is invoked with an lvalue (a named variable or an expression that evaluates to an lvalue).
Object a; // a is lvalue Object b = std::move(a);
This time, the move template instantiates to:
// move with [T = Object&]
remove_reference<Object&>::type&& move(Object& && arg)
{ return static_cast<remove_reference<Object&>::type&&>(arg); }Again, remove_reference
Object&& move(Object& && arg) { return static_cast<Object&&>(arg); }At first glance, this seems counterintuitive since we've passed an lvalue. However, C++11 introduces the concept of reference collapsing, which modifies the interpretation of reference syntax:
Under these rules, Object& && effectively behaves as Object&, an ordinary lvalue reference that can bind to lvalues.
Therefore, the final form of the function becomes:
Object&& move(Object& arg) { return static_cast<Object&&>(arg); }In essence, std::move() casts the lvalue argument to an rvalue reference, enabling perfect forwarding regardless of the input type.
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