C++11新特性之 std::forward(完美轉發)

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。
（也許“轉換”二字用的不是很准確）

如果我們要避免這種轉換呢？
我們需要一種方法能按照參數原來的類型轉發到另一個函數中，這才完美，我們稱之為完美轉發。

std::forward就可以保存參數的左值或右值特性。

因為是這樣描述的：
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
void wrapper(T&& arg) 
{
    foo(std::forward(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.

看一段網站上的代碼(http://en.cppreference.com/w/cpp/utility/forward)：

#include 
#include 
#include 
#include 

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
    B(T1&& t1, T2&& t2, T3&& t3) :
        a1_{std::forward(t1)},
        a2_{std::forward(t2)},
        a3_{std::forward(t3)}
    {
    }

private:
    A a1_, a2_, a3_;
};

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

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

int main()
{   
    auto p1 = make_unique1(2); // rvalue
    int i = 1;
    auto p2 = make_unique1(i); // lvalue

    std::cout << "B\n";
    auto t = make_unique(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

最後，記住：
不管是T&&、左值引用、右值引用，std::forward都會按照原來的類型完美轉發。