[wwwdocs] Document allocator_traits<A>::rebind_alloc assertion with GCC 13
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Pushed to wwwdocs.
---
htdocs/gcc-13/porting_to.html | 60 +++++++++++++++++++++++++++++++++++
1 file changed, 60 insertions(+)
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On Thu, 2 Mar 2023, Jonathan Wakely via Gcc-patches wrote:
> Pushed to wwwdocs.
Thank you!
> +If an allocator type <tt>Alloc<T></tt>
Note that HTML 5 complains about the use of <tt> and we are using <code>
instead.
I just pushed the following patch addressing that. (The diff looks a bit
bigger due to changes re line breaks.)
Gerald
commit 5a75fbda8c3c647b2ef659ffe67a031ee957abe6
Author: Gerald Pfeifer <gerald@pfeifer.com>
Date: Fri Mar 3 23:41:36 2023 +0100
gcc-13: Use <code> instead of <tt>
diff --git a/htdocs/gcc-13/porting_to.html b/htdocs/gcc-13/porting_to.html
index 953e1453..733bb254 100644
--- a/htdocs/gcc-13/porting_to.html
+++ b/htdocs/gcc-13/porting_to.html
@@ -150,8 +150,8 @@ previous behavior.
GCC 13 now checks that allocators used with the standard library
can be "rebound" to allocate memory for a different type,
as required by the allocator requirements in the C++ standard.
-If an allocator type <tt>Alloc<T></tt>
-cannot be correctly rebound to another type <tt>Alloc<U></tt>,
+If an allocator type <code>Alloc<T></code>
+cannot be correctly rebound to another type <code>Alloc<U></code>,
you will get an error like this:
</p>
@@ -161,26 +161,27 @@ you will get an error like this:
<p>
The assertion checks that rebinding an allocator to its own value type is a
-no-op, which will be true if its <tt>rebind</tt> member is defined correctly.
+no-op, which will be true if its <code>rebind</code> member is defined correctly.
If rebinding it to its own value type produces a different type,
then the allocator cannot be used with the standard library.
</p>
<p>
-The most common cause of this error is an allocator type <tt>Alloc<T></tt>
-that derives from <tt>std::allocator<T></tt> but does not provide its own
-<tt>rebind</tt> member. When the standard library attempts to rebind the
-allocator using <tt>Alloc<T>::rebind<U></tt> it finds the
-<tt>std::allocator<T>::rebind<U></tt> member from the base class,
-and the result is <tt>std::allocator<U></tt> instead of
-<tt>Alloc<U></tt>.
+The most common cause of this error is an allocator type
+<code>Alloc<T></code> that derives from
+<code>std::allocator<T></code> but does not provide its own
+<code>rebind</code> member. When the standard library attempts to rebind the
+allocator using <code>Alloc<T>::rebind<U></code> it finds the
+<code>std::allocator<T>::rebind<U></code> member from the base
+class, and the result is <code>std::allocator<U></code> instead of
+<code>Alloc<U></code>.
</p>
<p>
-The solution is to provide a correct <tt>rebind</tt> member as shown below.
-A converting constructor must also be provided, so that that an
-<tt>Alloc<U></tt> can be constructed from an <tt>Alloc<T></tt>,
-and vice versa:
+The solution is to provide a correct <code>rebind</code> member as shown
+below. A converting constructor must also be provided, so that that an
+<code>Alloc<U></code> can be constructed from an
+<code>Alloc<T></code>, and vice versa:
</p>
<pre><code>
template<class T>
@@ -197,9 +198,10 @@ class Alloc
</code></pre>
<p>
-Since C++20, there is no <tt>rebind</tt> member in <tt>std::allocator</tt>,
-so deriving your own allocator types from <tt>std::allocator</tt> is simpler
-and doesn't require the derived allocator to provide its own <tt>rebind</tt>.
+Since C++20, there is no <code>rebind</code> member in
+<code>std::allocator</code>, so deriving your own allocator types from
+<code>std::allocator</code> is simpler and doesn't require the derived
+allocator to provide its own <code>rebind</code>.
For compatibility with previous C++ standards, the member should still be
provided. The converting constructor is still required even in C++20.
</p>
@@ -144,5 +144,65 @@ done in the i387 floating point stack or are spilled from it.
The <code>-fexcess-precision=fast</code> option can be used to request the
previous behavior.
+<h3 id="alloc-rebind">allocator_traits<A>::rebind_alloc<A::value_type> must be A</h3>
+
+<p>
+GCC 13 now checks that allocators used with the standard library
+can be "rebound" to allocate memory for a different type,
+as required by the allocator requirements in the C++ standard.
+If an allocator type <tt>Alloc<T></tt>
+cannot be correctly rebound to another type <tt>Alloc<U></tt>,
+you will get an error like this:
+</p>
+
+<pre>
+.../bits/alloc_traits.h:70:31: error: static assertion failed: allocator_traits<A>::rebind_alloc<A::value_type> must be A
+</pre>
+
+<p>
+The assertion checks that rebinding an allocator to its own value type is a
+no-op, which will be true if its <tt>rebind</tt> member is defined correctly.
+If rebinding it to its own value type produces a different type,
+then the allocator cannot be used with the standard library.
+</p>
+
+<p>
+The most common cause of this error is an allocator type <tt>Alloc<T></tt>
+that derives from <tt>std::allocator<T></tt> but does not provide its own
+<tt>rebind</tt> member. When the standard library attempts to rebind the
+allocator using <tt>Alloc<T>::rebind<U></tt> it finds the
+<tt>std::allocator<T>::rebind<U></tt> member from the base class,
+and the result is <tt>std::allocator<U></tt> instead of
+<tt>Alloc<U></tt>.
+</p>
+
+<p>
+The solution is to provide a correct <tt>rebind</tt> member as shown below.
+A converting constructor must also be provided, so that that an
+<tt>Alloc<U></tt> can be constructed from an <tt>Alloc<T></tt>,
+and vice versa:
+</p>
+<pre><code>
+template<class T>
+class Alloc
+{
+ Alloc();
+ <b>
+ template<class U> Alloc(const Alloc<U>);
+
+ template<class U> struct rebind { using other = Alloc<U>; };
+ </b>
+ // ...
+};
+</code></pre>
+
+<p>
+Since C++20, there is no <tt>rebind</tt> member in <tt>std::allocator</tt>,
+so deriving your own allocator types from <tt>std::allocator</tt> is simpler
+and doesn't require the derived allocator to provide its own <tt>rebind</tt>.
+For compatibility with previous C++ standards, the member should still be
+provided. The converting constructor is still required even in C++20.
+</p>
+
</body>
</html>