Constructors That Use an Rvalue Reference (C++11)

C++11 adds move semantics to the string class. As described in Chapter 18, this involves adding a move constructor, which uses an rvalue reference instead of an lvalue reference:

basic_string(basic_string&& str) noexcept;

This constructor is invoked when the actual argument is a temporary object:

string one("din");     // C-style string constructor
string two(one);       // copy constructor – one is an lvalue
string three(one+two); // move constructor, sum is an rvalue

As discussed in Chapter 18, the intent is that string three takes ownership of the object constructed by operator+() rather than copying the object and then letting the original be destroyed.

The second rvalue constructor additionally allows you to specify an allocator:

basic_string(const basic_string&& str, const Allocator&);

The following relationships hold after either of these two constructors is called:

• The data() method returns a pointer to an allocated copy of the array whose first element is pointed to by str.data().

• The size() method returns the value of str.size().

• The capacity() method returns a value at least as large as size().