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 ([csignal.syn]).
template <class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept; template <class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
Returns: static_cast<T&&>(t).
Remarks: If the second form is instantiated with an lvalue reference type, the program is ill-formed.
[ Example:
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); int i = 2;
shared_ptr<A> sp2 = factory<A>(i, 1.414); }
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> constexpr remove_reference_t<T>&& move(T&& t) noexcept;
Returns: static_cast<remove_reference_t<T>&&>(t).
[ Example:
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&); A(A&&); };
void g() {
A a;
shared_ptr<A> sp1 = factory<A>(a); shared_ptr<A> sp1 = factory<A>(std::move(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;
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