On 28/04/21 17:57 +0100, Jonathan Wakely wrote:
On 07/04/21 17:59 +0100, Jonathan Wakely wrote:
On 07/04/21 13:46 +0100, Jonathan Wakely wrote:
On 07/04/21 15:41 +0300, Ville Voutilainen via Libstdc++ wrote:
On Wed, 7 Apr 2021 at 15:31, Jonathan Wakely via Libstdc++
<libstd...@gcc.gnu.org> wrote:
I propose that we deprecate the constructors for C++11/14/17/20 in
stage 1, and do not support them at all in C++23 mode once P1951 is
supported. I have a patch which I'll send in stage 1 (it also uses
C++20 concepts to simplify std::pair and fix PR 97930).
After a period of deprecation we could remove them, and support P1951
for -std=gnu++11/14/17/20 too so that {} continues to work.
The proposal sounds good to me.
Thanks. I've created https://gcc.gnu.org/PR99957 so I don't forget.
Here's a patch to implement it, for stage 1.
libstdc++: Deprecate non-standard std::pair constructors [PR 99957]
This deprecates the non-standard std::pair constructors that support
construction from an rvalue and a literal zero used as a null pointer
constant. We can't just add the deprecated attribute to those
constructors, because they're currently used by correct code when they
are a better match than the constructors required by the standard e.g.
int i = 0;
const int j = 0;
std::pair<int, int> p(i, j); // uses pair(U1&&, const int&)
This patch adjusts the parameter types and constraints of those
constructors so that they only get used for literal zeros, and the
pair(U1&&, U2&&) constructor gets used otherwise. Once they're only used
for initializations that should be ill-formed we can add the deprecated
attribute.
The deprecated attribute is used to suggest that the user code uses
nullptr, which avoids the problem of 0 deducing as int instead of a null
pointer constant.
I've pushed this to trunk, after testing on powerpc64le-linux.
And here's the wwwdocs patch announcing the deprecation.
Pushed to wwwdocs.
commit 24151c175c08222c268d9c37cc8fa255e2b2384c
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Wed Apr 28 17:59:50 2021 +0100
Document deprecation of non-standard std::pair constructors for GCC 12
diff --git a/htdocs/gcc-12/changes.html b/htdocs/gcc-12/changes.html
index e0ac986e..912fc59b 100644
--- a/htdocs/gcc-12/changes.html
+++ b/htdocs/gcc-12/changes.html
@@ -30,6 +30,13 @@ a work-in-progress.</p>
<!-- .................................................................. -->
<h2>Caveats</h2>
<ul>
+ <li>
+ Two non-standard <tt>std::pair</tt> constructors have been deprecated.
+ These allowed the use of an rvalue and a literal <tt>0</tt> to construct
+ a pair containing a move-only type and a pointer. The <tt>nullptr</tt>
+ keyword should be used to initialize the pointer member instead of a
+ literal <tt>0</tt>, as this is portable to other C++ implementations.
+ </li>
<li>...</li>
</ul>
