我们也要时刻清醒，有时候右值会转为左值，左值会转为右值。
（也许“转换”二字用的不是很准确）
如果我们要避免这种转换呢？
我们需要一种方法能按照参数原来的类型转发到另一个函数中，这才完美，我们称之为完美转发。
std::forward就可以保存参数的左值或右值特性。
因为是这样描述的：
When used according to the following recipe in a function template, forwards the argument to another function with the value category it had when passed to the calling function.
例子：
1 template<class T>
2 void wrapper(T&& arg)
3 {
4 foo(std::forward<T>(arg)); // Forward a single argument.
5 }
If a call to wrapper() passes an rvalue std::string, then T is deduced to std::string (not std::string&, const std::string&, or std::string&&), and std::forward ensures that an rvalue reference is passed to foo.
If a call to wrapper() passes a const lvalue std::string, then T is deduced to const std::string&, and std::forward ensures that a const lvalue reference is passed to foo.
If a call to wrapper() passes a non-const lvalue std::string, then T is deduced to std::string&, and std::forward ensures that a non-const lvalue reference is passed to foo.
看一段网站上的代码(http://en.cppreference.com/w/cpp/utility/forward)：
1 #include <iostream>
2 #include <memory>
3 #include <utility>
4 #include <array>
5
6 struct A {
7 A(int&& n) { std::cout << "rvalue overload, n=" << n << "\n"; }
8 A(int& n) { std::cout << "lvalue overload, n=" << n << "\n"; }
9 };
10
11 class B {
12 public:
13 template<class T1, class T2, class T3>
14 B(T1&& t1, T2&& t2, T3&& t3) :
15 a1_{std::forward<T1>(t1)},
16 a2_{std::forward<T2>(t2)},
17 a3_{std::forward<T3>(t3)}
18 {
19 }
20
21 private:
22 A a1_, a2_, a3_;
23 };
24
25 template<class T, class U>
26 std::unique_ptr<T> make_unique1(U&& u)
27 {
28 return std::unique_ptr<T>(new T(std::forward<U>(u)));
29 }
30
31 template<class T, class... U>
32 std::unique_ptr<T> make_unique(U&&... u)
33 {
34 return std::unique_ptr<T>(new T(std::forward<U>(u)...));
35 }
36
37 int main()
38 {
39 auto p1 = make_unique1<A>(2); // rvalue
40 int i = 1;
41 auto p2 = make_unique1<A>(i); // lvalue
42
43 std::cout << "B\n";
44 auto t = make_unique<B>(2, i, 3);
45 }
46 //输出：
47 rvalue overload, n=2
48 lvalue overload, n=1
49 B
50 rvalue overload, n=2
51 lvalue overload, n=1
52 rvalue overload, n=3
最后，记住：
不管是T&&、左值引用、右值引用，std::forward都会按照原来的类型完美转发。
转自：https://blog.csdn.net/wangshubo1989/article/details/50485951
原文地址：https://www.cnblogs.com/zl1991/p/9078617.html