OK
On Mon, 21 Jul 2025, 12:10 Luc Grosheintz, <luc.groshei...@gmail.com> wrote:
> Previously, the default ctor of mdspan was never noexcept, even if all
> members of mdspan were nothrow default constructible.
>
> This commit makes mdspan conditionally nothrow default constructible.
> A similar strengthening happens in libc++.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/mdspan (mdspan::mdspan): Make default ctor
> conditionally noexcept.
> * testsuite/23_containers/mdspan/mdspan.cc: Add tests.
>
> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> ---
> libstdc++-v3/include/std/mdspan | 12 +++---
> .../testsuite/23_containers/mdspan/mdspan.cc | 37 +++++++++++++++++++
> 2 files changed, 42 insertions(+), 7 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index f71141f43a9..772ee32db06 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -1114,11 +1114,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> constexpr
> mdspan()
> requires (rank_dynamic() > 0)
> - && is_default_constructible_v<data_handle_type>
> + && is_default_constructible_v<data_handle_type>
> && is_default_constructible_v<mapping_type>
> - && is_default_constructible_v<accessor_type>
> - : _M_accessor(), _M_mapping(), _M_handle()
> - { }
> + && is_default_constructible_v<accessor_type> = default;
>
> constexpr
> mdspan(const mdspan& __other) = default;
> @@ -1306,9 +1304,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> stride(rank_type __r) const { return _M_mapping.stride(__r); }
>
> private:
> - [[no_unique_address]] accessor_type _M_accessor;
> - [[no_unique_address]] mapping_type _M_mapping;
> - [[no_unique_address]] data_handle_type _M_handle;
> + [[no_unique_address]] accessor_type _M_accessor = accessor_type();
> + [[no_unique_address]] mapping_type _M_mapping = mapping_type();
> + [[no_unique_address]] data_handle_type _M_handle =
> data_handle_type();
> };
>
> template<typename _CArray>
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> index 942f6d96dca..6ffddd11e95 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> @@ -4,6 +4,7 @@
> #include <testsuite_hooks.h>
> #include "int_like.h"
> #include "layout_like.h"
> +#include <stdexcept>
>
> constexpr auto dyn = std::dynamic_extent;
>
> @@ -114,6 +115,27 @@ test_class_properties_all()
> return true;
> }
>
> +template<typename T>
> + class ThrowingDefaultAccessor
> + {
> + public:
> + using element_type = T;
> + using reference = element_type&;
> + using data_handle_type = element_type*;
> + using offset_policy = ThrowingDefaultAccessor;
> +
> + ThrowingDefaultAccessor() noexcept(false)
> + { }
> +
> + reference
> + access(data_handle_type p, size_t i) const
> + { return p[i]; }
> +
> + typename offset_policy::data_handle_type
> + offset(data_handle_type p, size_t i) const
> + { return p + i; }
> + };
> +
> constexpr bool
> test_default_ctor()
> {
> @@ -130,6 +152,18 @@ test_default_ctor()
> return true;
> }
>
> +template<template<typename T> typename Accessor, bool Expected>
> + constexpr void
> + test_nothrow_default_ctor()
> + {
> + using Extents = std::extents<int, dyn>;
> + using Layout = std::layout_left;
> + using MDSpan = std::mdspan<double, Extents, Layout, Accessor<double>>;
> +
> + static_assert(std::is_default_constructible_v<MDSpan>);
> + static_assert(std::is_nothrow_default_constructible_v<MDSpan> ==
> Expected);
> + }
> +
> constexpr bool
> test_from_other()
> {
> @@ -683,6 +717,9 @@ main()
> test_default_ctor();
> static_assert(test_default_ctor());
>
> + test_nothrow_default_ctor<std::default_accessor, true>();
> + test_nothrow_default_ctor<ThrowingDefaultAccessor, false>();
> +
> test_from_other();
> static_assert(test_from_other());
>
> --
> 2.50.0
>
>
