[PATCH 1/2] Add gcc/make-unique.h

[David Malcolm <dmalcolm@redhat.com>]

On Fri, 2022-07-08 at 22:16 +0100, Jonathan Wakely wrote:
> On Fri, 8 Jul 2022 at 21:47, David Malcolm via Gcc <gcc@gcc.gnu.org>
> wrote:
> > 
> > std::unique_ptr is C++11, and I'd like to use it in the
> > gcc/analyzer
> > subdirectory, at least.  The following patch eliminates a bunch of
> > "takes ownership" comments and manual "delete" invocations in favor
> > of simply using std::unique_ptr.
> > 
> > The problem is that the patch makes use of std::make_unique, but
> > that
> > was added in C++14.
> > 
> > I've heard that it's reasonably easy to reimplement
> > std::make_unique,
> > but I'm not sure that my C++11 skills are up to it.
> 
> You know we have an implementation of std::make_unique in GCC, with a
> GCC-compatible licence that you can look at, right? :-)
> 
> But it's not really necessary. There are only two reasons to prefer
> make_unique over just allocating an object with new and constructing
> a
> unique_ptr from it:
> 
> 1) avoid a "naked" new in your code (some coding styles like this,
> but
> it's not really important as long as the 'delete' is managed
> automatically by unique_ptr).
> 
> 2) exception-safety when allocating multiple objects as args to a
> function, see https://herbsutter.com/gotw/_102/ for details.
> Irrelevant for GCC, because we build without exceptions.

[moving from gcc to gcc-patches mailing list]

Also, I *think* it's a lot less typing, since I can write just:

  std::make_unique<name_of_type_which_could_be_long> (args)

rather than

  std::unique_ptr<name_of_type_which_could_be_long> (new name_of_type_which_could_be_long (args));

> 
> 
> 
> > Is there:
> > (a) an easy way to implement a std::make_unique replacement
> >     (e.g. in system.h? what to call it?), or
> 
> If you don't care about using it to create unique_ptr<T[]> arrays,
> it's trivial:
> 
>   template<typename T, typename... Args>
>     inline typename std::enable_if<!std::is_array<T>::value,
> std::unique_ptr<T>>::type
>     make_unique(Args&&... args)
>     { return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
> }
> 
> To add the overload that works for arrays is a little trickier.

Thanks!

I tried adding it to gcc/system.h, but anything that uses it needs to
have std::unique_ptr declared, which meant forcibly including <memory>
from gcc/system.h

So instead, here's a patch that adds a new gcc/make-unique.h header,
containing just the template decl above (in the root namespace, rather
than std::, which saves a bit more typing).

I've successfully bootstrapped&regression-tested a version of my earlier
analyzer patch that uses this patch (see patch 2 of the kit, which has
lots of usage examples).

OK for trunk?

Dave



This patch adds gcc/make-unique.h, containing a minimal C++11
implementation of make_unique (std::make_unique is C++14).

gcc/ChangeLog:
	* make-unique.h: New file.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>
---
 gcc/make-unique.h | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)
 create mode 100644 gcc/make-unique.h

diff --git a/gcc/make-unique.h b/gcc/make-unique.h
new file mode 100644
index 00000000000..c99c5328545
--- /dev/null
+++ b/gcc/make-unique.h
@@ -0,0 +1,41 @@
+/* Minimal implementation of make_unique for C++11 compatibility.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#ifndef GCC_MAKE_UNIQUE
+#define GCC_MAKE_UNIQUE
+
+/* This header uses std::unique_ptr, but <memory> can't be directly
+   included due to issues with macros.  Hence it must be included from
+   system.h by defining INCLUDE_MEMORY in any source file using it.  */
+
+#ifndef INCLUDE_MEMORY
+# error "You must define INCLUDE_MEMORY before including system.h to use make-unique.h"
+#endif
+
+/* Minimal implementation of make_unique for C++11 compatibility
+   (std::make_unique is C++14).  */
+
+template<typename T, typename... Args>
+inline typename std::enable_if<!std::is_array<T>::value, std::unique_ptr<T>>::type
+make_unique(Args&&... args)
+{
+  return std::unique_ptr<T> (new T (std::forward<Args> (args)...));
+}
+
+#endif /* ! GCC_MAKE_UNIQUE */
-- 
2.26.3