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

[Jason Merrill <jason@redhat.com>]

On 10/26/22 16:40, David Malcolm via Gcc-patches wrote:
> 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?

OK.

> 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 */