# “Perfect backwarding”

Tonight at CppCon, Ezra Chung gave a lightning talk on what he called “perfect forwarding and perfect backwarding.” Perfect forwarding, as we all know, is when you receive an argument from above and pass it downward with the same value category; Ezra’s “perfect backwarding,” then, is when you receive a return value from below and pass it upward with the same value category.

Here are our test cases for perfect forwarding, and also for perfect backwarding:

void taker(int&);
void taker(const int&);
void taker(int&&);
void taker(const int&&);

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

int giver(index_constant<0>);
int& giver(index_constant<1>);
const int& giver(index_constant<2>);
int&& giver(index_constant<3>);
const int&& giver(index_constant<4>);

template<int N>
decltype(auto) perfect_backward() {
return giver(index_constant<N>{});
}


We already kind of see our test cases breaking down, in that perfect_backward<0>() returns a different type from perfect_backward<3>(), but both of those types eagerly turn into int&& when passed as function arguments. (Perfect forwarding cannot perfectly forward prvalues!)

But where Ezra’s code really gets tricky is if you want to name the return value (possibly so you can do something else with it before returning it).

template<int N>
decltype(auto) imperfect_backward_alpha() {
decltype(auto) result = giver(index_constant<N>{});
printf("%d\n", result);
return std::forward<decltype(result)>(result);
}


This imperfect_backward_alpha works in all cases except when result’s type is a non-reference type. In that case, std::forward<int>(result) has type int&&, and so we return a soon-to-be-dangling reference. (GCC catches this bug — hooray!)

In his lightning talk, Ezra suggested

template<int N>
decltype(auto) imperfect_backward_beta() {
decltype(auto) result = giver(index_constant<N>{});
printf("%d\n", result);
return decltype(result)(result);
}


This works, for int, but it has bad behavior for std::string. In the non-reference-type case there, we end up with the moral equivalent of

std::string result = giver(index_constant<0>{});
return std::string(result);


which of course not only inhibits copy elision but also makes a copy of result.

What we really want to do, to get “perfect backwarding” of all return values — even prvalue return values — is more like this:

template<int N>
decltype(auto) perfect_backward() {
decltype(auto) result = giver(index_constant<N>{});
printf("%d\n", result);
if constexpr (std::is_reference_v<decltype(result)>) {
return decltype(result)(result);
} else {
return result;
}
}


I don’t think there’s any shorter way to write this in C++17.

But it occurs to me that we can eliminate all this wacky metaprogramming by adopting a minor tweak to David Stone’s P0527 “Implicit move from rvalue references in return statements.” All we’d need to do is make an additional (orthogonal) change to the Standard’s current wording so that this example also did an implicit move:

std::string&& example() {
std::string&& result = something();
return result;
}


Right now, that’s ill-formed: the expression result is an lvalue expression (because result is a named variable), and so return result is attempting to bind the lvalue result to an rvalue reference std::string&&, and that doesn’t work (yes, even though decltype(result) is std::string&&).

If we could somehow change the Standard so that our example() was legal C++, then Ezra’s “perfect backwarding” problem would have a simple solution:

template<int N>
auto perfect_backward_hypothetical()
-> decltype(giver(index_constant<N>{}))
{
decltype(auto) result = giver(index_constant<N>{});
printf("%d\n", result);
return result;
}


I suspect there’s some reason this change would be too scary to ever actually implement in C++, but I can’t think of the reason off the top of my head.

(Perfect forwarding of prvalue expressions is left as an exercise for the reader.)

Posted 2018-09-25