NAME

std::move - std::move

Synopsis

Defined in header <utility>
template< class T > (since C++11)
typename std::remove_reference<T>::type&& move( T&& t ) noexcept; (until C++14)
template< class T > (since C++14)
constexpr std::remove_reference_t<T>&& move( T&& t ) noexcept;

std::move is used to indicate that an object t may be "moved from", i.e. allowing
the efficient transfer of resources from t to another object.

In particular, std::move produces an xvalue expression that identifies its argument
t. It is exactly equivalent to a static_cast to an rvalue reference type.

Parameters

t - the object to be moved

Return value

static_cast<typename std::remove_reference<T>::type&&>(t)

Notes

The functions that accept rvalue reference parameters (including move constructors,
move assignment operators, and regular member functions such as
std::vector::push_back) are selected, by overload resolution, when called with
rvalue arguments (either prvalues such as a temporary object or xvalues such as the
one produced by std::move). If the argument identifies a resource-owning object,
these overloads have the option, but aren't required, to move any resources held by
the argument. For example, a move constructor of a linked list might copy the
pointer to the head of the list and store nullptr in the argument instead of
allocating and copying individual nodes.

Names of rvalue reference variables are lvalues and have to be converted to xvalues
to be bound to the function overloads that accept rvalue reference parameters, which
is why move constructors and move assignment operators typically use std::move:

// Simple move constructor
A(A&& arg) : member(std::move(arg.member)) // the expression "arg.member" is lvalue
{}
// Simple move assignment operator
A& operator=(A&& other) {
member = std::move(other.member);
return *this;
}

One exception is when the type of the function parameter is rvalue reference to type
template parameter ("forwarding reference" or "universal reference"), in which case
std::forward is used instead.

Unless otherwise specified, all standard library objects that have been moved from
are placed in a "valid but unspecified state", meaning the object's class invariants
hold (so functions without preconditions, such as the assignment operator, can be
safely used on the object after it was moved from):

std::vector<std::string> v;
std::string str = "example";
v.push_back(std::move(str)); // str is now valid but unspecified
str.back(); // undefined behavior if size() == 0: back() has a precondition !empty()
if (!str.empty())
str.back(); // OK, empty() has no precondition and back() precondition is met

str.clear(); // OK, clear() has no preconditions

Also, the standard library functions called with xvalue arguments may assume the
argument is the only reference to the object; if it was constructed from an lvalue
with std::move, no aliasing checks are made. However, self-move-assignment of
standard library types is guaranteed to place the object in a valid (but usually
unspecified) state:

std::vector<int> v = {2, 3, 3};
v = std::move(v); // the value of v is unspecified

Example

// Run this code

#include <iomanip>
#include <iostream>
#include <utility>
#include <vector>
#include <string>

int main()
{
std::string str = "Salut";
std::vector<std::string> v;

// uses the push_back(const T&) overload, which means
// we'll incur the cost of copying str
v.push_back(str);
std::cout << "After copy, str is " << std::quoted(str) << '\n';

// uses the rvalue reference push_back(T&&) overload,
// which means no strings will be copied; instead, the contents
// of str will be moved into the vector. This is less
// expensive, but also means str might now be empty.
v.push_back(std::move(str));
std::cout << "After move, str is " << std::quoted(str) << '\n';

std::cout << "The contents of the vector are { " << std::quoted(v[0])
<< ", " << std::quoted(v[1]) << " }\n";
}

Possible output:

After copy, str is "Salut"
After move, str is ""
The contents of the vector are { "Salut", "Salut" }