On Wed, 28 May 2025, Tomasz Kaminski wrote:
>
>
> On Tue, May 27, 2025 at 7:08 PM Patrick Palka <ppa...@redhat.com> wrote:
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk/15?
>
> The 'volatile' issue from that PR Will be fixed in a separate patch as
> operator[] isn't the only operation that's affected.
>
> -- >8 --
>
> The const lvalue operator[] overload wasn't properly forwarding the key
> type to the generic overload.
>
> PR libstdc++/120432
>
> libstdc++-v3/ChangeLog:
>
> * include/std/flat_map (_Flat_map_base::operator[]): Correct
> forwarding from the const lvalue key overload.
> * testsuite/23_containers/flat_map/1.cc (test08): New test.
> * testsuite/23_containers/flat_multimap/1.cc (test08): New test.
> ---
> libstdc++-v3/include/std/flat_map | 2 +-
> libstdc++-v3/testsuite/23_containers/flat_map/1.cc | 10 ++++++++++
> .../testsuite/23_containers/flat_multimap/1.cc | 10 ++++++++++
> 3 files changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/std/flat_map
> b/libstdc++-v3/include/std/flat_map
> index 6593988d213c..4d9ced1e8191 100644
> --- a/libstdc++-v3/include/std/flat_map
> +++ b/libstdc++-v3/include/std/flat_map
> @@ -1142,7 +1142,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // element access
> mapped_type&
> operator[](const key_type& __x)
> - { return operator[]<const key_type>(__x); }
> + { return operator[]<const key_type&>(__x); }
>
> Given that the operator[] that we are forading to is implemented as:
> { return try_emplace(std::forward<_Key2>(__x)).first->second; }
> I would just call try_emplace directly:
Good point, the implementation is a simple one-liner either way, and it
addresses the volatile key issue. Like so?
-- >8 --
Subject: [PATCH] libstdc++: Fix flat_map::operator[] for const lvalue keys
[PR120432]
The const lvalue operator[] overload wasn't properly forwarding the key
type to the generic overload, causing a hard error for const keys.
This patch fixes this by making the non-template overloads call
try_emplace directly instead, which means we can remove the non-standard
same_as constraint on the generic overload.
PR libstdc++/120432
libstdc++-v3/ChangeLog:
* include/std/flat_map (flat_map::operator[]): Make the
non-template overloads call try_emplace directly. Remove
non-standard same_as constraint on the template overload.
* testsuite/23_containers/flat_map/1.cc (test08): New test.
---
libstdc++-v3/include/std/flat_map | 6 +++---
libstdc++-v3/testsuite/23_containers/flat_map/1.cc | 10 ++++++++++
2 files changed, 13 insertions(+), 3 deletions(-)
diff --git a/libstdc++-v3/include/std/flat_map
b/libstdc++-v3/include/std/flat_map
index 6593988d213c..5f9a2eda1939 100644
--- a/libstdc++-v3/include/std/flat_map
+++ b/libstdc++-v3/include/std/flat_map
@@ -1142,14 +1142,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// element access
mapped_type&
operator[](const key_type& __x)
- { return operator[]<const key_type>(__x); }
+ { return try_emplace(__x).first->second; }
mapped_type&
operator[](key_type&& __x)
- { return operator[]<key_type>(std::move(__x)); }
+ { return try_emplace(std::move(__x)).first->second; }
template<typename _Key2>
- requires same_as<remove_cvref_t<_Key2>, _Key> ||
__transparent_comparator<_Compare>
+ requires __transparent_comparator<_Compare>
mapped_type&
operator[](_Key2&& __x)
{ return try_emplace(std::forward<_Key2>(__x)).first->second; }
diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
index a9690208b09f..2af516410279 100644
--- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
+++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
@@ -253,6 +253,15 @@ test07()
VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4}}) );
}
+void
+test08()
+{
+ // PR libstdc++/120432 - flat_map operator[] is broken for const lvalue keys
+ std::flat_map<int, int> m;
+ const int k = 42;
+ m[k] = 0;
+}
+
int
main()
{
@@ -266,4 +275,5 @@ main()
test05();
test06();
test07();
+ test08();
}
--
2.49.0.654.g845c48a16a
