Don’t forward things that aren’t forwarding references

Some readers of yesterday’s post (“A deduction guide for Foo(Ts..., Ts...), 2023-05-26) may wonder about the part where I wrote:

template<class... Ts>
class Foo {
  explicit Foo(Ts... as, Ts... bs)
    : a_(static_cast<Ts&&>(as)...),
      b_(static_cast<Ts&&>(bs)...) {}

  std::tuple<Ts...> a_;
  std::tuple<Ts...> b_;

Since we all know that std::forward<T>(t) is synonymous with static_cast<T&&>(t), why did I choose static_cast above instead of writing std::forward<Ts>(as)...?

The answer is not simply that I prefer core-language features over library facilities, and it’s not that I’m worried about forward’s compile-time performance. Now, it’s true that the Standard Library could get along fine without std::forward: Whenever you use std::forward<T>(t) on a forwarding reference T&& t, it’s always doing exactly the same thing as static_cast<T&&>(t). One could rewrite all one’s std::forwards as casts. But we generally don’t do this, because the specific spelling std::forward gives the reader useful information: “I’m forwarding this forwarding-reference parameter!” It’s greppable, and somewhat self-explanatory — much more so than a cast, anyway. We use std::forward when we are, semantically, doing perfect forwarding.

If you use std::forward only for perfect-forwarding, then I can take your program and mechanically search-and-replace every instance of std::forward<T>(t) into decltype(t)(t), and it’ll still compile and do exactly the same thing. I’ve actually done this for libc++, by search-and-replacing std::forward<T>(x) into _LIBCPP_FWD(x) and then compiling with various macro-definitions of _LIBCPP_FWD. Of course this works only if you never do anything “weird” via std::forward. If you shoehorn it in places where it is physically correct but semantically incorrect, this mechanical transformation won’t work at all. For example, if you write:

std::unique_ptr<int> oops() {
  auto p = std::make_unique<int>(42);
  return std::forward<std::unique_ptr<int>>(p);

This return statement is physically valid: It takes the lvalue p, abuses std::forward to cast it to unique_ptr&&, and then constructs the return slot using unique_ptr’s move-constructor. But it’s not semantically correct: We aren’t doing perfect forwarding here, so we shouldn’t use std::forward. In fact what we meant was simply

  return p;

In my experience, programmers new to move semantics will tend to sprinkle their code with std::move and std::forward until it “works,” ending up with a program that doesn’t look nearly as clean as it should, and works by accident, if at all. (Programmers new to pointers will do the same with * and &.) It helps immensely if we can give simple concrete rules for when to use move and forward, and when not to.

  • Use std::move(x) on the last use of an object x, to signify your willingness to transfer ownership of that object’s resources. Otherwise, don’t use move.

  • Use std::forward<X>(x) when perfect-forwarding a function parameter of forwarding-reference type X&&. (This, also, is a transfer of ownership, and must be the last use of x.) Otherwise, don’t use forward.

Just as the guideline against “pass-by-const-value” allows us to quickly detect that there’s a bug in

auto plus(const std::string s, const std::string& t) {
    return s + t;

the guideline against “forwarding anything that’s not a forwarding reference” allows us to quickly detect that there’s some kind of mistake in

template<class T>
void bar(std::vector<T>& c, int i) {

(a snippet seen in a real std-proposals thread from 2018). We might not know quite what the bug is, yet; but we know something’s wrong, because the programmer is using forward where no forwarding reference exists.

The flip side — the price I must pay to gain this amazing bug-finding benefit — is that I mustn’t write forward where no forwarding reference exists. Not even when, as in yesterday’s post, it contingently happens to work out to the right physical behavior in all cases. In those situations, I avoid forward.

Use std::forward only when perfect-forwarding.

See also:

Posted 2023-05-27