is any difference between std::forward<T> and std::forward<decltype(t)>?

问题

are these functions equivalent?

template <class T>
void foo(T && t)
{
    bar(std::forward<T>(t));
}

template <class T>
void foo2(T && t)
{
    bar(std::forward<decltype(t)>(t));
}

template <class T>
void foo3(T && t)
{
    bar(std::forward(t));
}

if they are, can I always use this macro for perfect forwarding?

#define MY_FORWARD(var) std::forward<decltype(var)>(var)

or just use

bar(std::forward(t));

I believe foo2 and foo3 are same, but I found people are always use forward like foo, is any reason to explicitly write the type?

I understand that T and T&& are two different types, but I think std::forward<T> and std::forward<T&&> always give the same result?

---

Edit:

the reason I want to use macro is I want to save some typing on following C++1y code, I have many similar code in different places

#define XLC_FORWARD_CAPTURE(var) var(std::forward<decltype(var)>(var))
#define XLC_MOVE_CAPTURE(var) var(std::move(var))

template <class T, class U>
auto foo(T && func, U && para )
{
    auto val = // some calculation
    return [XLC_FORWARD_CAPTURE(func),
            XLC_FORWARD_CAPTURE(para),
            XLC_MOVE_CAPTURE(val)](){
              // some code use val
              func(std::forward<U>(para)); 
          };
}

回答1:

Are these functions two equivalent?

Yes, they are equivalent. decltype(t) is the same as T&&, and when used with std::forward, there is no difference between T and T&&, regardless what T is.

Can I always use this macro for perfect forwarding?

Yes, you can. If you want to make your code unreadable and unmaintainable, then do so. But I strongly advise against it. On the one hand, you gain basically nothing from using this macro. And on the other hand, other developers have to take a look at the definition to understand it, and it can result in subtle errors. For example adding additional parentheses won't work:

MY_FORWARD((t))

In contrast, the form with decltype is perfectly valid. In particular, it is the preferred way of forwarding parameters from generic lambda expressions, because there are no explicit type parameters:

[](auto&& t) { foobar(std::forward<decltype(t)>(t)); }

---

I ignored the 3rd variant with std::forward(t), because it isn't valid.

---

Update: Regarding your example: You can use call-by-value instead of call-by-reference for the function template foo. Then you can use std::move instead of std::forward. This adds two additional moves to the code, but no additional copy operations. On the other hand, the code becomes much cleaner:

template <class T, class U>
auto foo(T func, U para)
{
    auto val = // some calculation
    return [func=std::move(func),para=std::move(para),val=std::move(val)] {
        // some code use val
        func(std::move(para)); 
    };
}

回答2:

The accepted answer does not solve the problem in title completely.

A macro argument preserves the type of the expression. A forwarding parameter in a template does not. This means t in foo2 (as a forwarding function parameter) has the type T&& (because this is the forwarding template parameter), but it can be something different when the macro is in other contexts. For example:

using T = int;
T a = 42;
T&& t(std::move(a));
foo(MY_FORWARD(t)); // Which foo is instantiated?

Note here t is not an xvalue, but an lvalue. With std::forward<T>(t), which is equivalent to std::forward<int>(t), t would be forwarded as an lvalue. However, with MY_FORWARD(t), which is equivalent to std::forward<int&&>(t), t would be forwarded as an xvalue. This contextual-dependent difference is sometime desired when you have to deal with some declared variables with rvalue reference types (not forwarding paramter even they may look like similar in syntax).

来源：https://stackoverflow.com/questions/23321028/is-any-difference-between-stdforwardt-and-stdforwarddecltypet