Document #: P3074R2
Date: 2024-02-13
Project: Programming Language C++
Audience: EWG
Reply-to: Barry Revzin

1 Revision History

[P3074R0] originally proposed the function std::start_lifetime(p). This revision adds a new section discussing the uninitialized storage problem, which motivates a change in design to instead propose std::uninitialized<T>.

[P3074R1] changed to propose std::uninitialized<T> and was discussed in an EWG telecon. There, the suggestion was made to make this a language feature, which this revision discusses and argues against. Also re-spelled std::uninitialized<T> to be a union instead of a class containing an anonymous union.

2 Introduction

Consider the following example:

template <typename T, size_t N>
struct FixedVector {
    union U { constexpr U() { } constexpr ~U() { } T storage[N]; };
    U u;
    size_t size = 0;

    // note: we are *not* constructing storage
    constexpr FixedVector() = default;

    constexpr ~FixedVector() {

    constexpr auto push_back(T const& v) -> void {
        std::construct_at( + size, v);

constexpr auto silly_test() -> size_t {
    FixedVector<std::string, 3> v;
    v.push_back("some sufficiently longer string");
    return v.size;
static_assert(silly_test() == 1);

This is basically how any static/non-allocating/in-place vector is implemented: we have some storage, that we definitely do not value initialize and then we steadily construct elements into it.

The problem is that the above does not work (although there is implementation divergence - MSVC and EDG accept it and GCC did accept it even up to 13.2, but GCC trunk and Clang reject).

Getting this example to work would allow std::inplace_vector ([P0843R9]) to simply work during constexpr time for all times (instead of just trivial ones), and was a problem briefly touched on in [P2747R0].

2.1 The uninitialized storage problem

A closely related problem to the above is: how do you do uninitialized storage? The straightforward implementation would be to do:

template <class T>
struct BufferStorage {
    alignas(T) unsigned char buffer[sizeof(T)];

    // accessors

This approach generally works, but it has two limitations:

  1. it cannot work in constexpr and that’s likely a fundamental limitation that will never change, and
  2. it does not quite handle overlapping objects correctly.

What I mean by the second one is basically given this structure:

struct Empty { };

struct Sub : Empty {
    BufferStorage<Empty> buffer_storage;

If we initialize the Empty that buffer_storage is intended to have, then Sub has two subobjects of type Empty. But the compiler doesn’t really… know that, and doesn’t adjust them accordingly. As a result, the Empty base class subobject and the Empty initialized in buffer_storage are at the same address, which violates the rule that all objects of one type are at unique addresses.

An alternative approach to storage is to use a union:

template <class T>
struct UnionStorage {
  union { T value; };

  // accessors

struct Sub : Empty {
    UnionStorage<Empty> union_storage;

Here, now the compiler knows for sure there is an Empty in union_storage and will lay out the types appropriately. See also gcc bug 112591.

So it seems that the UnionStorage approach is strictly superior: it will work in constexpr and it lays out overlapping types properly. But it has limitations of its own. As with the FixedVector example earlier, you cannot just start the lifetime of value. But also in this case we run into the union rules for special member functions: a special member of a union, by default, is either trivial (if that special member for all alternatives is trivial) or deleted (otherwise). Which means that UnionStorage<std::string> has both its constructor and destructor deleted.

We can work around this by simply adding an empty constructor and destructor (as shown earlier as well):

template <class T>
struct UnionStorage2 {
  union U { U() { } ~U() { } T value; };
  U u;

  // accessors

This is a fundamentally weird concept since U there has a destructor that does nothing (and given that this is a class to be used for uninitialized storage), it should do nothing - that’s correct. But that destructor still isn’t trivial. And it turns out there is still a difference between “destructor that does nothing” and “trivial destructor”:

Trivially Destructible
Non-trivially Destructible
struct A { };

auto alloc_a(int n) -> A* { return new A[n]; }
auto del(A* p) -> void { delete [] p; }
struct B { ~B() { } };

auto alloc_b(int n) -> B* { return new B[n]; }
auto del(B* p) -> void { delete [] p; }
        movsx   rdi, edi
        jmp     operator new[](unsigned long)

        test    rdi, rdi
        je      .L3
        jmp     operator delete[](void*)
        movabs  rax, 9223372036854775800
        push    rbx
        movsx   rbx, edi
        cmp     rax, rbx
        lea     rdi, [rbx+8]
        mov     rax, -1
        cmovb   rdi, rax
        call    operator new[](unsigned long)
        mov     QWORD PTR [rax], rbx
        add     rax, 8
        pop     rbx

        test    rdi, rdi
        je      .L9
        mov     rax, QWORD PTR [rdi-8]
        sub     rdi, 8
        lea     rsi, [rax+8]
        jmp     operator delete[](void*, unsigned long)

That’s a big difference in code-gen, due to the need to put a cookie in the allocation so that the corresponding delete[] knows how many elements there so that their destructors (even though they do nothing!) can be invoked.

While the union storage solution solves some language problems for us, the buffer storage solution can lead to more efficient code - because StorageBuffer<T> is always trivially trivially destructible. It would be nice if he had a good solution to all of these problems - and that solution was also the most efficient one.

3 Design Space

A previous revision of this paper [P3074R0] talked about three potential solutions to this problem:

  1. a library solution (add a std::uninitialized<T>)
  2. just make it work (change the union rules to implicitly start the lifetime of the first alternative, if it’s an implicit-lifetime type)
  3. add a library function to explicitly start lifetime

That paper proposed a new function std::start_lifetime(p) that was that third option. However, with the addition of the overlapping subobjects problem and the realization that the union solution has overhead compared to the buffer storage solution, it would be more desirable to solve both problems in one go.

Now, (2) is no longer a meaningful option because we can’t really “just make it work” since attempting to change the union rules with regards to trivial destruction would be quite a language change and an ABI break.

Similarly, (3) doesn’t really address the storage problem. In order to make the union solution work out, we’d need to add another way to tell the compiler that we want our constructor and destructor to be trivial - not deleted - regardless of the alternatives. Perhaps that ends up being something like this:

template <class T>
struct UnionStorageFuture {
    union { [[uninitialized]] T value; };
    constexpr UnionStorageFuture() { std::start_lifetime(&value); }

However, now we need two features - some way to mark the alternative and some way to start its lifetime. That, inherently, doesn’t seem like a great solution.

Which seems to leave only one solution to the problem, which is easier to use anyway:

3.1 A library type: std::uninitialized<T>

We could introduce another magic library type, std::uninitialized<T>, with an interface like:

template <typename T>
struct uninitialized {
    union { T value; };

As basically a better version of std::aligned_storage. Here is storage for a T, that implicitly begins its lifetime if T is an implicit-lifetime-type, but otherwise will not actually initialize it for you - you have to do that yourself. Likewise it will not destroy it for you, you have to do that yourself too. This type would be specified to always be trivially default constructible and trivially destructible. It would be trivially copyable if T is trivially copyable, otherwise not copyable.

std::inplace_vector<T, N> would then have a std::uninitialized<T[N]> and go ahead and std::construct_at (or, with [P2747R1], simply placement-new) into the appropriate elements of that array and everything would just work.

Because the language would recognize this type, this would also solve the overlapping objects problem.

3.2 A language annotation

During the EWG telecon in January 2023, the suggestion was made that instead of a magic library type like std::uninitialized<T>, we could instead have some kind of language annotation to achieve the same effect.

For example:

template <typename T, size_t N>
struct FixedVector {
    // as a library feature
    std::uninitialized<T[N]> lib;

    //as a language feature, something like this
    for storage T lang[N];
    T storage[N] = for lang;
    uninitialized T lang[N];

    size_t size = 0;

The advantage of the language syntax is that you can directly use lang - you would placement new onto lang[0], you read from lang[1], etc, whereas with the library syntax you have to placement new onto lib.value[0] and read from lib.value[1], etc.

However, an uninitialized object of type T really isn’t the same thing as a T. decltype(lang) would have to be T, any kind of (imminent) reflection over this type would give you a T. But there might not actually be a T there yet, it behaves like a union { T; } rather than a T, so spelling it T strikes me as misleading.

We would have to ensure that all the other member-wise algorithms we have today (the special member functions and the comparisons) use the “uninitialized T” meaning rather than the T meaning. And with reflection, that also means all future member-wise algorithms would have to account for this also - rather than rejecting unions.

The syntactic benefits of the language syntax are nice, but this is a rarely used type for specific situations - so having slightly longer syntax (and really, lib.value is not especially cumbersome) is not only not a big downside here but could even be viewed as a benefit.

So despite the fact that there was consensus to prefer a language solution over a library solution:

5 4 4 2 1

Having had more time to consider it, I believe the library solution to be superior.

3.3 Existing Practice

There are two similar features in other languages that I’m aware of.

Rust has MaybeUninit<T> which is similar to what’s proposed here as std::uninitialized<T>.

Kotlin has a lateinit var language feature, which is similar to some kind of language annotation (although additionally allows for checking whether it has been initialized, which the language feature would not provide).

4 Wording

Add to 20.2.2 [memory.syn]:

namespace std {
  // ...
  // [obj.lifetime], explicit lifetime management
  template<class T>
    T* start_lifetime_as(void* p) noexcept;                                         // freestanding
  template<class T>
    const T* start_lifetime_as(const void* p) noexcept;                             // freestanding
  template<class T>
    volatile T* start_lifetime_as(volatile void* p) noexcept;                       // freestanding
  template<class T>
    const volatile T* start_lifetime_as(const volatile void* p) noexcept;           // freestanding
  template<class T>
    T* start_lifetime_as_array(void* p, size_t n) noexcept;                         // freestanding
  template<class T>
    const T* start_lifetime_as_array(const void* p, size_t n) noexcept;             // freestanding
  template<class T>
    volatile T* start_lifetime_as_array(volatile void* p, size_t n) noexcept;       // freestanding
  template<class T>
    const volatile T* start_lifetime_as_array(const volatile void* p,               // freestanding
                                          size_t n) noexcept;

+ template<class T>
+   union uninitialized;                                                           // freestanding

With corresponding wording in 20.2.6 [obj.lifetime]:

9 The union uninitialized is suitable for storage for an object of type T that is not initially initialized.

template<class T>
union uninitialized {
  T value;

  constexpr uninitialized();
  constexpr uninitialized(const uninitialized&);
  constexpr uninitialized& operator=(const uninitialized&);
  constexpr ~uninitialized();

10 uninitialized<T> is a trivially default constructible and trivially destructible type.

11 Note 1: An object of type T and the value subobject of uninitialized<T> have distinct addresses ([intro.object]) — end note ]

constexpr uninitialized();

12 Effects: If T is an implicit-lifetime type, begins the lifetime of value. Otherwise, none. Note 2: The constructor of T, if any, is not called — end note ]

constexpr uninitialized(const uninitialized&);
constexpr uninitialized& operator=(const uninitialized&);

13 If T is a trivially copyable type, then uninitialized<T> is a trivially copyable type. Otherwise, uninitialized<T> is not copyable.

constexpr ~uninitialized();

14 Effects: None. Note 3: The destructor of T, if any, is not called — end note ]

5 References

[P0843R9] Gonzalo Brito Gadeschi, Timur Doumler, Nevin Liber, David Sankel. 2023-09-14. inplace_vector.

[P2747R0] Barry Revzin. 2022-12-17. Limited support for constexpr void*.

[P2747R1] Barry Revzin. 2023-12-10. constexpr placement new.

[P3074R0] Barry Revzin. 2023-12-15. constexpr union lifetime.

[P3074R1] Barry Revzin. 2023. std::uninitialized<T>.