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### 3626. Is `std::basic_stacktrace` required to use contiguous storage?

Section: 20.21.4.1 [stacktrace.basic.overview] Status: New Submitter: Jiang An Opened: 2021-10-23 Last modified: 2021-10-27

Priority: 3

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Discussion:

Currently `std::basic_stacktrace` has an exposition-only `std::vector` member for storing its elements. According to 16.3.3.5 [objects.within.classes]/3, it seems that it is effectively required that elements of a `std::basic_stacktrace` are contiguously stored. However, the implication seems not used otherwhere. The iterator type of a `std::basic_stacktrace` is only required to be random access iterator.

IMO if it is required that `std::basic_stacktrace` uses contiguous storage, we should explicitly strengthen some requirements, perhaps a the member function data should be added.

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll.
"The problem here is that a handful of member functions (`operator[]`, `at`, perhaps `begin`) expose references to the vector elements directly, which can be read to require contiguity. We should rephrase the members at issue to not do that."

Proposed resolution:

This wording is relative to N4901.

> [Drafting note: The proposed wording below contains also conditional changes, it is therefore depending upon a decision]

1. Modify 20.21.4.1 [stacktrace.basic.overview] as indicated:

   > -1- The class template `basic_stacktrace` satisfies the requirements of an allocator-aware container (Table 80 [tab:container.alloc.req]), a sequence container (22.2.3 [sequence.reqmts]), a contiguous container, and a reversible container (22.2.1 [container.requirements.general]) except that […]

2. Modify 20.21.4.3 [stacktrace.basic.obs] as indicated:

   >     using const_iterator = implementation-defined;

   > > -1- The type models `random_access_iteratorcontiguous_iterator` (23.3.4.13 [iterator.concept.random.access]23.3.4.14 [iterator.concept.contiguous]) and meets the Cpp17RandomAccessIterator requirements (23.3.5.7 [random.access.iterators]).

Optional additional changes (the following parts are proposed only if `data()` is wanted)

3. Modify 20.21.4.1 [stacktrace.basic.overview], class template `basic_stacktrace` synopsis, as indicated:

   >     […]
   >     const_reference operator[](size_type) const;
   >     const_reference at(size_type) const;

   >     const stacktrace_entry* data() const noexcept;

   >     // 20.21.4.4 [stacktrace.basic.cmp], comparisons
   >     […]

4. Modify 20.21.4.3 [stacktrace.basic.obs] as indicated:

   >     const_reference at(size_type frame_no) const;

   > > -13- […]

   > > -14- […]

   >     const stacktrace_entry* data() const noexcept;

   > > -?- Returns: `frames_.data()`.