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4064. Clarify thatstd::launder is not needed when using the result ofstd::memcpy

Section: 27.5.1[cstring.syn]Status:WPSubmitter: Jan SchultkeOpened: 2024-04-05Last modified: 2024-11-28

Priority:3

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Discussion:

int x = 0;alignas(int) std::byte y[sizeof(int)];int z = *static_cast<int*>(std::memcpy(y, &x, sizeof(int)));

This example should be well-defined, even without the use ofstd::launder.std::memcpy implicitly creates anint insidey, andhttps://www.iso-9899.info/n3047.html#7.26.2.1p3 states that

Thememcpy function returns the value of [the destination operand].

In conjunction with 27.5.1[cstring.syn] p3, this presumably means thatstd::memcpy returns a pointer to the (first) implicitly-created object, and no use ofstd::launder is necessary.

The wording should be clarified to clearly support this interpretation or reject it.

[2024-06-24; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]:

This wording is relative toN4971.

1. Modify 27.5.1[cstring.syn] as indicated:

   -3- The functionsmemcpy andmemmove are signal-safe (17.14.5[support.signal]). Both functions implicitly create objects (6.8.2[intro.object]) in the destination region of storage immediately prior to copying the sequence of characters to the destination.Both functions return a pointer to a suitable created object.

[St. Louis 2024-06-26; CWG suggested improved wording]

[St. Louis 2024-06-28; LWG: move to Ready]

[Wrocław 2024-11-18; approved by Core (again)]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative toN4981.

1. Modify 27.5.1[cstring.syn] as indicated:

   -3-The functionsmemcpy andmemmove are signal-safe(17.14.5[support.signal]).BothEach of these functions implicitlycreatecreates objects (6.8.2[intro.object])in the destination region of storage immediately prior to copyingthe sequence of characters to the destination.Each of these functions returns a pointer to a suitable created object,if any, otherwise the value of the first parameter.