[patch] Use perfect forwarding in std::function's invokers

[patch] Use perfect forwarding in std::function's invokers

[Jonathan Wakely]

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As pointed out in http://stackoverflow.com/q/26543242/981959 our
std::function does more copying/moving than necessary. The solution is
to use perfect forwarding for the internal invoker functions, even if
function<R(Args...)::operator() and the target function take their
arguments by value.

Tested x86_64-linux, committed to trunk.

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commit 9c733bb92dd60c416731c049ee58ae56c7e0117f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Oct 29 18:04:16 2014 +0000

    Use perfect forwarding in std::function invokers.
    
    	* include/std/functional: (_Function_base::_Function_base()): Use
    	nullptr instead of literal zero.
    	(function::operator=(nullptr_t)): Likewise.
    	(_Function_handler::_M_invoke): Use perfect forwarding for _ArgTypes.
    	(function::_Invoker_type): Likewise.
    	* testsuite/20_util/function/invoke/forwarding.cc: New.

diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index bed1eea..5bc2730 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -1990,7 +1990,7 @@ _GLIBCXX_HAS_NESTED_TYPE(result_type)
 	}
       };
 
-    _Function_base() : _M_manager(0) { }
+    _Function_base() : _M_manager(nullptr) { }
 
     ~_Function_base()
     {
@@ -2019,7 +2019,7 @@ _GLIBCXX_HAS_NESTED_TYPE(result_type)
 
     public:
       static _Res
-      _M_invoke(const _Any_data& __functor, _ArgTypes... __args)
+      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
       {
 	return (*_Base::_M_get_pointer(__functor))(
 	    std::forward<_ArgTypes>(__args)...);
@@ -2034,7 +2034,7 @@ _GLIBCXX_HAS_NESTED_TYPE(result_type)
 
      public:
       static void
-      _M_invoke(const _Any_data& __functor, _ArgTypes... __args)
+      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
       {
 	(*_Base::_M_get_pointer(__functor))(
 	    std::forward<_ArgTypes>(__args)...);
@@ -2049,7 +2049,7 @@ _GLIBCXX_HAS_NESTED_TYPE(result_type)
 
      public:
       static _Res
-      _M_invoke(const _Any_data& __functor, _ArgTypes... __args)
+      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
       {
 	return __callable_functor(**_Base::_M_get_pointer(__functor))(
 	      std::forward<_ArgTypes>(__args)...);
@@ -2064,7 +2064,7 @@ _GLIBCXX_HAS_NESTED_TYPE(result_type)
 
      public:
       static void
-      _M_invoke(const _Any_data& __functor, _ArgTypes... __args)
+      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
       {
 	__callable_functor(**_Base::_M_get_pointer(__functor))(
 	    std::forward<_ArgTypes>(__args)...);
@@ -2081,7 +2081,7 @@ _GLIBCXX_HAS_NESTED_TYPE(result_type)
 
      public:
       static _Res
-      _M_invoke(const _Any_data& __functor, _ArgTypes... __args)
+      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
       {
 	return std::mem_fn(_Base::_M_get_pointer(__functor)->__value)(
 	    std::forward<_ArgTypes>(__args)...);
@@ -2121,7 +2121,7 @@ _GLIBCXX_HAS_NESTED_TYPE(result_type)
       }
 
       static void
-      _M_invoke(const _Any_data& __functor, _ArgTypes... __args)
+      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
       {
 	std::mem_fn(_Base::_M_get_pointer(__functor)->__value)(
 	    std::forward<_ArgTypes>(__args)...);
@@ -2275,8 +2275,8 @@ _GLIBCXX_HAS_NESTED_TYPE(result_type)
 	if (_M_manager)
 	  {
 	    _M_manager(_M_functor, _M_functor, __destroy_functor);
-	    _M_manager = 0;
-	    _M_invoker = 0;
+	    _M_manager = nullptr;
+	    _M_invoker = nullptr;
 	  }
 	return *this;
       }
@@ -2395,7 +2395,7 @@ _GLIBCXX_HAS_NESTED_TYPE(result_type)
 #endif
 
     private:
-      typedef _Res (*_Invoker_type)(const _Any_data&, _ArgTypes...);
+      using _Invoker_type = _Res (*)(const _Any_data&, _ArgTypes&&...);
       _Invoker_type _M_invoker;
   };
 
diff --git a/libstdc++-v3/testsuite/20_util/function/invoke/forwarding.cc b/libstdc++-v3/testsuite/20_util/function/invoke/forwarding.cc
new file mode 100644
index 0000000..b6e1b47
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/function/invoke/forwarding.cc
@@ -0,0 +1,56 @@
+// Copyright (C) 2014 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library 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.
+
+// This library 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 this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++11" }
+
+#include <functional>
+#include <testsuite_hooks.h>
+
+struct Counted
+{
+  Counted() : count(0) { }
+  Counted(const Counted& c) : count(c.count + 1) { }
+  int count;
+};
+
+int func(Counted c) { return c.count; }
+
+std::function<int(Counted)> f = func;
+
+void
+test01()
+{
+  Counted c;
+  int n = f(c);
+  // 1 copy invoking function::operator() and 1 copy invoking func
+  VERIFY( n == 2 );
+}
+
+void
+test02()
+{
+  int n = f(Counted{});
+  // copy elided when invoking function::operator(), 1 copy invoking func
+  VERIFY( n == 1 );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}