22 General utilities library [utilities]

22.2 Utility components [utility]

22.2.4 Forward/move helpers [forward]

The library provides templated helper functions to simplify applying move semantics to an lvalue and to simplify the implementation of forwarding functions.
All functions specified in this subclause are signal-safe.
template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept; template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
Mandates: For the second overload, is_­lvalue_­reference_­v<T> is false.
Returns: static_­cast<T&&>(t).
[Example 1: template<class T, class A1, class A2> shared_ptr<T> factory(A1&& a1, A2&& a2) { return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2))); } struct A { A(int&, const double&); }; void g() { shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2; shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK }
In the first call to factory, A1 is deduced as int, so 2 is forwarded to A's constructor as an rvalue.
In the second call to factory, A1 is deduced as int&, so i is forwarded to A's constructor as an lvalue.
In both cases, A2 is deduced as double, so 1.414 is forwarded to A's constructor as an rvalue.
— end example]
template<class T, class U> [[nodiscard]] constexpr auto forward_like(U&& x) noexcept -> see below;
  • Let COPY_­CONST(A, B) be const B if A is a const type, otherwise B.
  • Let OVERRIDE_­REF(A, B) be remove_­reference_­t<B>&& if A is an rvalue reference type, otherwise B&.
  • Let V be OVERRIDE_REF(T&&, COPY_CONST(remove_reference_t<T>, remove_reference_t<U>))
Returns: static_­cast<V>(x).
Remarks: The return type is V.
[Example 2: struct accessor { vector<string>* container; decltype(auto) operator[](this auto&& self, size_t i) { return std::forward_like<decltype(self)>((*container)[i]); } }; void g() { vector v{"a"s, "b"s}; accessor a{&v}; string& x = a[0]; // OK, binds to lvalue reference string&& y = std::move(a)[0]; // OK, is rvalue reference string const&& z = std::move(as_const(a))[1]; // OK, is const&& string& w = as_const(a)[1]; // error: will not bind to non-const } — end example]
template<class T> constexpr remove_reference_t<T>&& move(T&& t) noexcept;
Returns: static_­cast<remove_­reference_­t<T>&&>(t).
[Example 3: template<class T, class A1> shared_ptr<T> factory(A1&& a1) { return shared_ptr<T>(new T(std::forward<A1>(a1))); } struct A { A(); A(const A&); // copies from lvalues A(A&&); // moves from rvalues }; void g() { A a; shared_ptr<A> sp1 = factory<A>(a); // “a'' binds to A(const A&) shared_ptr<A> sp2 = factory<A>(std::move(a)); // “a'' binds to A(A&&) }
In the first call to factory, A1 is deduced as A&, so a is forwarded as a non-const lvalue.
This binds to the constructor A(const A&), which copies the value from a.
In the second call to factory, because of the call std​::​move(a), A1 is deduced as A, so a is forwarded as an rvalue.
This binds to the constructor A(A&&), which moves the value from a.
— end example]
template<class T> constexpr conditional_t< !is_nothrow_move_constructible_v<T> && is_copy_constructible_v<T>, const T&, T&&> move_if_noexcept(T& x) noexcept;
Returns: std​::​move(x).