C++11新特性之 std::forward(完美轉發)

上篇博客對右值、右值引用都做了簡要介紹。

我們也要時刻清醒，有時候右值會轉為左值，左值會轉為右值。
（或許“轉換”二字用的不是非常准確）

假設我們要避免這樣的轉換呢？
我們須要一種方法能依照參數原來的類型轉發到還有一個函數中。這才完美，我們稱之為完美轉發。

std::forward就能夠保存參數的左值或右值特性。

由於是這樣描寫敘述的：
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.

樣例：

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.

看一段站點上的代碼(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>(2); // rvalue
    int i = 1;
    auto p2 = make_unique1<A>(i); // lvalue

    std::cout << "B\n";
    auto t = make_unique<B>(2, i, 3);
}
//輸出：
rvalue overload, n=2
lvalue overload, n=1
B
rvalue overload, n=2
lvalue overload, n=1
rvalue overload, n=3

最后，記住：
無論是T&&、左值引用、右值引用，std::forward都會依照原來的類型完美轉發。