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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
C++11新特性之 std::forward(完美转发)（转）
标签：main include fun wan AC shu ios 网站 int
原文地址：https://www.cnblogs.com/zl1991/p/9078617.html
