[PATCH v3] Add gcc/make-unique.h

[David Malcolm <dmalcolm@redhat.com>]

Changed in v3: added include of <type_traits>
v2: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604137.html
v1: https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598189.html

On Tue, 2022-07-12 at 07:48 +0100, Jonathan Wakely wrote:
> On Tue, 12 Jul 2022, 01:25 David Malcolm, <dmalcolm@redhat.com>
> wrote:
> 
> > 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).
> > 
> 
> Adding things to std isn't allowed anyway, so that's correct.
> 
> 
> > 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

[...snip...]

> > +#ifndef INCLUDE_MEMORY
> > +# error "You must define INCLUDE_MEMORY before including system.h
> > to use
> > make-unique.h"
> > +#endif
> > 
> 
> You also need <type_traits> for the enable_if and is_array traits.
> With
> libstdc++ that gets included by <memory> but that's guaranteed for
> other
> library implementations.
> 
> I don't know if that had the same kind of issues as other system
> headers or
> if it can just be included here.

I've added an include of <type_traits> in this version of the patch.

> 
> 
> +
> > +/* 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
> > 
> > 

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

Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu
in conjunction with a followup series of patches which use this
in dozens of places in the analyzer.

OK for trunk?

gcc/ChangeLog:
	* make-unique.h: New file.
---
 gcc/make-unique.h | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 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..c9a7d6ef6ce
--- /dev/null
+++ b/gcc/make-unique.h
@@ -0,0 +1,44 @@
+/* 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 <memory> must be included
+   from system.h by defining INCLUDE_MEMORY in any source file using
+   make-unique.h.  */
+
+#ifndef INCLUDE_MEMORY
+# error "You must define INCLUDE_MEMORY before including system.h to use make-unique.h"
+#endif
+
+#include <type_traits>
+
+/* 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