std.allocator: Regions are non-copyable, yet are passed around in examples

[dlangBugzillaToGithub]

dlang-bugzilla (@CyberShadow) reported this on 2018-05-10T11:21:07Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=18848

CC List

• andrei (@andralex)

Description

Here is an example from std.experimental.allocator.building_blocks.allocator_list.AllocatorList:

// Ouroboros allocator list based upon 4MB regions, fetched directly from
// mmap. All memory is released upon destruction.
alias A1 = AllocatorList!((n) => Region!MmapAllocator(max(n, 1024 * 4096)),
    NullAllocator);

The example features a lambda returning a Region.

However, Regions cannot be copied, as they have a destructor which deallocates everything. A comment in the implementation says:

    /*
    TODO: The postblit of $(D BasicRegion) should be disabled because such objects
    should not be copied around naively.
    */

(That's the only occurrence of BasicRegion in Phobos, so I assume that's just an old name for Region.)

Though NRVO may make the verbatim example code behave correctly, it is fragile, and small code modifications e.g. to work around issue 18848 can cause difficult-to-debug problems (due to dangling pointers in other Region instances).

[dlangBugzillaToGithub]

dlang-bugzilla (@CyberShadow) commented on 2018-05-15T23:03:19Z

https://github.com/dlang/phobos/pull/6509

See the comment there for more discussion.