Re: [PATCH 2/4] Introduce scoped_restore_signal

[Pedro Alves <pedro@palves.net>]

On 2021-06-15 12:14 p.m., Pedro Alves wrote:
> +/* Simple wrapper that allows lazy initialization / destruction of T.
> +   Slightly more efficient than gdb::optional, because it doesn't
> +   carry storage to track whether the object has been initialized.  */
> +template<typename T>
> +class lazy_init
> +{
> +public:
> +  void emplace ()
> +  {
> +    new (m_buf) T ();
> +  }
> +
> +  void reset ()
> +  {
> +    reinterpret_cast <T *> (m_buf)->~T ();
> +  }
> +
> +private:
> +  alignas (T) gdb_byte m_buf[sizeof (T)];
> +};
> +

I was looking at this again, and realized that a raw buffer like that isn't super
convenient when you're debugging gdb and want to inspect T (you'll need to cast).

So I made it a union instead (see below).  It has the added advantage that no cast
is necessary.

I'm going to push the series with this change.

diff --git c/gdbsupport/scoped_ignore_sigttou.h w/gdbsupport/scoped_ignore_sigttou.h
index 7b29f2b79a3..1fc8f80d7fd 100644
--- c/gdbsupport/scoped_ignore_sigttou.h
+++ w/gdbsupport/scoped_ignore_sigttou.h
@@ -34,16 +34,23 @@ class lazy_init
 public:
   void emplace ()
   {
-    new (m_buf) T ();
+    new (&m_u.obj) T ();
   }
 
   void reset ()
   {
-    reinterpret_cast <T *> (m_buf)->~T ();
+    m_u.obj.~T ();
   }
 
 private:
-  alignas (T) gdb_byte m_buf[sizeof (T)];
+  union u
+  {
+    /* Must define ctor/dtor if T has non-trivial ctor/dtor.  */
+    u () {}
+    ~u () {}
+
+    T obj;
+  } m_u;
 };