Move Constructors and std::move

We can avoid copying the strings by using two facilities introduced by the new library. First, several of the library classes, including string, define so-called “move constructors.” The details of how the string move constructor works—like any other detail about the implementation—are not disclosed. However, we do know that move constructors typically operate by “moving” resources from the given object to the object being constructed. We also know that the library guarantees that the “moved-from” string remains in a valid, destructible state. For string, we can imagine that each string has a pointer to an array of char. Presumably the string move constructor copies the pointer rather than allocating space for and copying the characters themselves.

The second facility we’ll use is a library function named move , which is defined in the utility header. For now, there are two important points to know about move. First, for reasons we’ll explain in § 13.6.1 (p. 532), when reallocate constructs the strings in the new memory it must call move to signal that it wants to use the string move constructor. If it omits the call to move the string the copy constructor will be used. Second, for reasons we’ll cover in § 18.2.3 (p. 798), we usually do not provide a using declaration (§ 3.1, p. 82) for move. When we use move, we call std::move, not move.