Double ampersand (&&) in C++11

The biggest difference between a C++03 reference (now called an lvalue reference in C++11) is that it can bind to an rvalue like a temporary without having to be const. Thus, this syntax is now legal

T&& r = T();

rvalue references primarily provide for the following

Move semantics

A move constructor and move assignment operator can now be defined that takes an rvalue reference instead of the usual const-lvalue reference. A move functions like a copy, except it is not obliged to keep the source unchanged; in fact, it usually modifies the source such that it no longer owns the owned resources. This is great for eliminating extraneous copies, especially in standard library implementations.

For example, a copy constructor might look like this

foo(foo const& other)
{
    this->length = other.length;
    this->ptr = new int[other.length];
    copy(other.ptr, other.ptr + other.length, this->ptr);
}

If this constructor was passed a temporary, the copy would be unnecessary because we know the temporary will just be destroyed; why not make use of the resources the temporary already allocated? In C++03, there’s no way to prevent the copy as we cannot determine we were passed a temporary. In C++11, we can overload a move constructor

foo(foo&& other)
{
   this->length = other.length;
   this->ptr = other.ptr;
   other.length = 0;
   other.ptr = nullptr;
}

Notice the big difference here: the move constructor actually modifies its argument. This would effectively “move” the temporary into the object being constructed, thereby eliminating the unnecessary copy.

The move constructor would be used for temporaries and for non-const lvalue references that are explicitly converted to rvalue references using the std::move function (it just performs the conversion). The following code both invoke the move constructor for f1 and f2

foo f1((foo())); // Move a temporary into f1; temporary becomes "empty"
foo f2 = std::move(f1); // Move f1 into f2; f1 is now "empty"

Perfect forwarding

rvalue references allow us to properly forward arguments for templated functions. Take for example this factory function:

template <typename T, typename A1>
std::unique_ptr<T> factory(A1& a1)
{
    return std::unique_ptr<T>(new T(a1));
}

If we called factory<foo>(5), the argument will be deduced to be int&, which will not bind to a literal 5, even if foo’s constructor takes an int. Well, we could instead use A1 const&, but what if foo takes the constructor argument by non-const reference? To make a truly generic factory function, we could have to overload factory on A1& and on A1 const &. That might be fine if factory takes 1 parameter type, but each additional parameter type would multiply the necessary overload set by 2. That’s very quickly unmaintainable.

rvalue references fix this problem by allowing the standard library to define a std::forward function that can properly forward lvalue/rvalue references.

This enables us to define the factory function like this

template <typename T, typename A1>
std::unique_ptr<T> factory(A1&& a1)
{
    return std::unique_ptr<T>(new T(std::forward<A1>(a1)));
}

Now the argument’s rvalue/lvaue-ness is preserved when passed to T’s constructor. That means the if factory is called with an rvalue, T’s constructor is called with an rvalue. If factory is called with an lvalue, T’s constructor is called with an lvalue. This improved function works because of one special rule

when the function parameter type is of the form T&& where T is a template parameter, and the function argument is an lvalue of type A, the type A& is used for template argument deduction.

Thus, we can use factory like so

auto p1 = factory<foo>(foo()); // calls foo(foo&&)
auto p2 = factory<foo>(*p1);   // calls foo(foo const&)

Important rvalue reference properties

• For overload resolution, lvalues prefer binding to lvalue reference and rvalues prefer binding to rvalue references. Hence why temporaries prefer invoking a move constructor/move assignment operator over a copy constructor/assignment operator.

• rvalue references will implicitly bind to rvalues and to temporaries that are the result of an implicit conversion. i.e. float f = 0f; int&& i = f; is well formed because float is implicitly convertible to int; the reference would be to a temporary that is the result of the conversion.

• Named rvalue reference are lvalues. Unnamed rvalue references are rvalues. This is important to understand why the std::move call is necessary in: foo&& r = foo(); foo f = std::move(r);

Reference

• https://stackoverflow.com/questions/5481539/what-does-t-double-ampersand-mean-in-c11