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### 2961. Bad postcondition for `set_default_resource`

Section: 20.12.4 [mem.res.global] Status: C++20 Submitter: Casey Carter Opened: 2017-05-09 Last modified: 2021-02-25

Priority: Not Prioritized

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Discussion:

The specification of `set_default_resource` in N4659 20.12.4 [mem.res.global] reads:

>     memory_resource* set_default_resource(memory_resource* r) noexcept;

> > -4- Effects: If `r` is non-null, sets the value of the default memory resource pointer to `r`, otherwise sets the default memory resource pointer to `new_delete_resource()`.

> > -5- Postconditions: `get_default_resource() == r`.

> > -6- Returns: The previous value of the default memory resource pointer.

> > -7- Remarks: […]

It is apparent that the effects specified in para 4 cannot — and indeed should not — achieve the postcondition specified in para 5 when `r` is null.

[2017-05-13, Tim comments]

This is the same issue as LWG 2522, which just missed the Fundamentals TS working draft used for the merge. A similar resolution (simply striking the Postconditions: paragraph) might be better.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4659.

> 1. Modify 20.12.4 [mem.res.global] as follows:

>    >     memory_resource* set_default_resource(memory_resource* r) noexcept;

>    > > -4- Effects: If `r` is non-null, sets the value of the default memory resource pointer to `r`, otherwise sets the default memory resource pointer to `new_delete_resource()`.

>    > > -5- Postconditions: If `r` is non-null, `get_default_resource() == r`. Otherwise, `get_default_resource() == new_delete_resource()`.

>    > > -6- Returns: The previous value of the default memory resource pointer.

>    > > -7- Remarks: […]

[2017-06-13, Casey Carter revises proposed wording]

I suggest to strike the Postconditions paragraph ([mem.res.global]/5) completely, as in LWG 2522

[2017-06-15, Moved to Tentatively Ready after 6 positive votes on c++std-lib]

Proposed resolution:

This wording is relative to N4659.

1. Modify 20.12.4 [mem.res.global] as follows:

   >     memory_resource* set_default_resource(memory_resource* r) noexcept;

   > > -4- Effects: If `r` is non-null, sets the value of the default memory resource pointer to `r`, otherwise sets the default memory resource pointer to `new_delete_resource()`.

   > > -5- Postconditions: `get_default_resource() == r`.

   > > -6- Returns: The previous value of the default memory resource pointer.

   > > -7- Remarks: […]