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3878. import std; should guarantee initialization of standard iostreams objects

Section: 31.4.2 [iostream.objects.overview] Status: C++23 Submitter: Tim Song Opened: 2023-02-09 Last modified: 2024-01-29

Priority: Not Prioritized

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In the old world, #include <iostream> behaves as if it defined a static-storage-duration ios_base::Init object, which causes the standard iostreams objects to be initialized (if necessary) on startup and flushed on shutdown.

But we don't include headers with import std;, so we need separate wording to provide this guarantee. The proposed resolution below was adapted from a suggestion by Mathias Stearn on the reflector.

[2023-02-09 Tim updates wording following LWG discussion]

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

  1. Modify 31.4.2 [iostream.objects.overview]p5 as indicated:

    -5- The results of including <iostream> in a translation unit shall be as if <iostream> defined an instance of ios_base::Init with static storage duration. Each C++ library module ( [std.modules]) in a hosted implementation shall behave as if it contains an interface unit that defines an unexported ios_base::Init variable with ordered initialization ( [basic.start.dynamic]).

    [Note ?: As a result, the definition of that variable is appearance-ordered before any declaration following the point of importation of a C++ library module. Whether such a definition exists is unobservable by a program that does not reference any of the standard iostream objects. — end note]