C++11新特性之 std::forward(完美转发)（转）

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：

 template<class T>
 void wrapper(T&& arg)
 {
     foo(std::forward<T>(arg)); // Forward a single argument.
 }

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)：

 #include <iostream>
 #include <memory>
 #include <utility>
 #include <array>

 struct A {
     A(int&& n) { std::cout << "rvalue overload, n=" << n << "\n"; }
     A(int& n)  { std::cout << "lvalue overload, n=" << n << "\n"; }
 };

 class B {
 public:
     template<class T1, class T2, class T3>
     B(T1&& t1, T2&& t2, T3&& t3) :
         a1_{std::forward<T1>(t1)},
         a2_{std::forward<T2>(t2)},
         a3_{std::forward<T3>(t3)}
     {
     }

 private:
     A a1_, a2_, a3_;
 };

 template<class T, class U>
 std::unique_ptr<T> make_unique1(U&& u)
 {
     return std::unique_ptr<T>(new T(std::forward<U>(u)));
 }

 template<class T, class... U>
 std::unique_ptr<T> make_unique(U&&... u)
 {
     return std::unique_ptr<T>(new T(std::forward<U>(u)...));
 }

 int main()
 {
     auto p1 = make_unique1<A>(); // rvalue
     int i = ;
     auto p2 = make_unique1<A>(i); // lvalue

     std::cout << "B\n";
     auto t = make_unique<B>(, i, );
 }
 //输出：
 rvalue overload, n=
 lvalue overload, n=
 B
 rvalue overload, n=
 lvalue overload, n=
 rvalue overload, n=

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