C++11中std::forward的使用 (转)

std::forward argument: Returns an rvalue reference to arg if arg is not an lvalue reference; If arg is an lvalue reference, the function returns arg without modifying its type.

std::forward:This is a helper function to allow perfect forwarding of arguments taken as rvalue references to deduced types, preserving any potential move semantics involved. 

std::forward<T>(u)有两个参数：T 与 u。当T为左值引用类型时，u将被转换为T类型的左值，否则u将被转换为T类型右值。如此定义std::forward是为了在使用右值引用参数的函数模板中解决参数的完美转发问题。

std::move是无条件的转为右值引用，而std::forward是有条件的转为右值引用，更准确的说叫做Perfect forwarding(完美转发)，而std::forward里面蕴含着的条件则是Reference Collapsing(引用折叠)。

std::move不move任何东西。std::forward也不转发任何东西。在运行时，他们什么都不做。不产生可执行代码，一个比特的代码也不产生。

std::move和std::forward只是执行转换的函数(确切的说应该是函数模板)。std::move无条件的将它的参数转换成一个右值，而std::forward当特定的条件满足时，才会执行它的转换。

std::move表现为无条件的右值转换，就其本身而已，它不会移动任何东西。 std::forward仅当参数被右值绑定时，才会把参数转换为右值。 std::move和std::forward在运行时不做任何事情。

下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：

#include "forward.hpp"
#include <utility>
#include <iostream>
#include <memory>
#include <string>

//////////////////////////////////////////////
// reference: http://en.cppreference.com/w/cpp/utility/forward
struct A {
    A(int&& n) { std::cout << "rvalue overload, n=" << n << "
"; }
    A(int& n)  { std::cout << "lvalue overload, 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 test_forward1()
{
    auto p1 = make_unique1<A>(2); // rvalue
    int i = 1;
    auto p2 = make_unique1<A>(i); // lvalue

    std::cout << "B
";
    auto t = make_unique<B>(2, i, 3);

    return 0;
}

////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/utility/forward/
// function with lvalue and rvalue reference overloads:
void overloaded(const int& x) { std::cout << "[lvalue]"; }
void overloaded(int&& x) { std::cout << "[rvalue]"; }

// function template taking rvalue reference to deduced type:
template <class T> void fn(T&& x) {
    overloaded(x);                   // always an lvalue
    overloaded(std::forward<T>(x));  // rvalue if argument is rvalue
}

int test_forward2()
{
    int a;

    std::cout << "calling fn with lvalue: ";
    fn(a);
    std::cout << '
';

    std::cout << "calling fn with rvalue: ";
    fn(0);
    std::cout << '
';

    return 0;
}

//////////////////////////////////////////////////////
// reference: http://stackoverflow.com/questions/8526598/how-does-stdforward-work
template<class T>
struct some_struct{
    T _v;
    template<class U>
    some_struct(U&& v) : _v(static_cast<U&&>(v)) {} // perfect forwarding here
    // std::forward is just syntactic sugar for this
};

int test_forward3()
{
    /* remember the reference collapsing rules(引用折叠规则):
    前者代表接受类型，后者代表进入类型，=>表示引用折叠之后的类型，即最后被推导决断的类型
    TR   R

    T&   &->T&   // lvalue reference to cv TR -> lvalue reference to T
    T&   &&->T&  // rvalue reference to cv TR -> TR (lvalue reference to T)
    T&&  &->T&   // lvalue reference to cv TR -> lvalue reference to T
    T&&  &&->T&& // rvalue reference to cv TR -> TR (rvalue reference to T) */

    some_struct<int> s1(5);
    // in ctor: '5' is rvalue (int&&), so 'U' is deduced as 'int', giving 'int&&'
    // ctor after deduction: 'some_struct(int&& v)' ('U' == 'int')
    // with rvalue reference 'v' bound to rvalue '5'
    // now we 'static_cast' 'v' to 'U&&', giving 'static_cast<int&&>(v)'
    // this just turns 'v' back into an rvalue
    // (named rvalue references, 'v' in this case, are lvalues)
    // huzzah, we forwarded an rvalue to the constructor of '_v'!

    // attention, real magic happens here
    int i = 5;
    some_struct<int> s2(i);
    // in ctor: 'i' is an lvalue ('int&'), so 'U' is deduced as 'int&', giving 'int& &&'
    // applying the reference collapsing rules yields 'int&' (& + && -> &)
    // ctor after deduction and collapsing: 'some_struct(int& v)' ('U' == 'int&')
    // with lvalue reference 'v' bound to lvalue 'i'
    // now we 'static_cast' 'v' to 'U&&', giving 'static_cast<int& &&>(v)'
    // after collapsing rules: 'static_cast<int&>(v)'
    // this is a no-op, 'v' is already 'int&'
    // huzzah, we forwarded an lvalue to the constructor of '_v'!

    return 0;
}

////////////////////////////////////////////////////
// reference: https://oopscenities.net/2014/02/01/c11-perfect-forwarding/
void sum(int a, int b)
{
    std::cout << a + b << std::endl;
}

void concat(const std::string& a, const std::string& b)
{
    std::cout<< a + b << std::endl;
}

void successor(int a, int& b)
{
    b = ++a;
}

template <typename PROC, typename A, typename B>
void invoke(PROC p, A&& a, B&& b)
{
    p(std::forward<A>(a), std::forward<B>(b));
}

int test_forward4()
{
    invoke(sum, 10, 20);
    invoke(concat, "Hello", "world");
    int s = 0;
    invoke(successor, 10, s);
    std::cout << s << std::endl;

    return 0;
}

GitHub：https://github.com/fengbingchun/Messy_Test