在C ++ 0x中完美转发使reference_wrapper不推荐使用？

问题描述

照常，代码第一：

#include <functional>

using namespace std;
using namespace std::tr1;

void f(int& r) { r++; }

template<class F, class P> void g1(F f, P t) { f(t); }
template<class F, class P> void g2(F f, P&& t) { f(forward<P>(t)); }

int main()
{
    int i = 0;

    g1(f, ref(i)); // old way, ugly way
    g2(f, i); // new way, elegant way
}

在C ++ 98中，通过模板函数完成向前参数的一种很好的方法。因此，C ++的古茹发明了ref和cref来实现这个目标。

In C++ 98, we don't have a nice way to pefect forward parameters through template functions. So the C++ gurus invented ref and cref to achieve that aim.

现在我们已经有了r值引用和完美的转发，这意味着ref和cref like应该被弃用？

Now that we have had r-value reference and perfect forwarding, does it mean that ref and cref and the like should be deprecated?

推荐答案

引用包装器仍然有用。这是关于存储的东西的情况。例如，使用引用包装器可以使std :: make_tuple和std :: thread创建引用一些参数而不是复制它们的对象：

Reference wrappers are still useful. This is the case when it's about storing things. For example, with reference wrappers you can make std::make_tuple and std::thread create objects which refer to some argument instead of copying them:

class abstract_job
{
public:
    virtual ~abstract_job() {}
    virtual void run() = 0;
};

class thread
{
public:
    template<class Fun, class... Args>
    thread(Fun&& f, Args&&... args)
    {
        typedef typename decay<Fun>::type fun_type;
        typedef decltype( make_tuple(forward<Args>(args)...) ) tuple_type;
        unique_ptr<abstract_job> ptr (new my_job<fun_type,tuple_type>(
            forward<Fun>(fun),
            make_tuple(forward<Args>(args)...)
        ));
        // somehow pass pointer 'ptr' to the new thread
        // which is supposed to invoke ptr->run();
    }
    ...
};

...

void foo(const int&);

int main()
{
   thread t (foo, 42); // 42 is copied and foo is invoked 
   t.join()            // with a reference to this copy
   int i = 23;
   thread z (foo, std::cref(i)); // foo will get a reference to i
   z.join();
}

请记住，

make_tuple(std::ref(i))  // yields a tuple<int&>
make_tuple(         i )  // yields a tuple<int>

干杯！
s

Cheers!s

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