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@kaizen_nagoya(Dr. Kiyoshi Ogawa)

22.2.4 Forward/move helpers [forward] C++N4910:2022 (523) p666b.cpp

Posted at

はじめに(Introduction)

N4910 Working Draft, Standard for Programming Language C++

n4910は、ISO/IEC JTC1 SC22 WG21の作業原案(Working Draft)です。
公式のISO/IEC 14882原本ではありません。
ISO/IEC JTC1 SC22 WG21では、可能な限り作業文書を公開し、幅広い意見を求めています。
一連の記事はコード断片をコンパイルできる形にする方法を検討してコンパイル、リンク、実行して、規格案の原文と処理系(g++, Clang++)との違いを確認し、技術内容を検討し、ISO/IEC JTC1 SC22 WG21にフィードバックするために用います。
また、CERT C++, MISRA C++等のコーディング標準のコード断片をコンパイルする際の参考にさせていただこうと考えています。CERT C++, MISRA C++が標準化の動きとの時間的なずれがあれば確認できれば幸いです。また、boostライブラリとの関連、Linux OS, TOPPERSカーネル、g++(GCC), clang++(LLVM）との関係も調査中です。
何か、抜け漏れ、耳より情報がありましたらおしらせくださると幸いです。

<この項は書きかけです。順次追記します。>

背景(back ground)

C/C++でコンパイルエラーが出ると、途方にくれることがしばしばあります。
何回かに１回は、該当するエラーが検索できます。
ただ、条件が違っていて、そこでの修正方法では目的を達成しないこともしばしばです。いろいろな条件のコンパイルエラーとその対応方法について、広く記録することによって、いつか同じエラーに遭遇した時にやくに立つことを目指しています。

この半年の間で、三度、自分のネットでの記録に助けられたことがあります。
また過去に解決できなかった記録を１０種類以上、最近になって解決できたことがあります。それは、主に次の３つの情報に基づいています。

https://stackoverflow.com
https://cpprefjp.github.io
http://ja.cppreference.com/

また
https://researchmap.jp/joub9b3my-1797580/#_1797580
に記載したサイトのお世話になっています。

作業方針(sequence)

Clang++では-std=c++03, C++2bの2種類
g++では-std=c++03, c++2bの２種類
でコンパイルし、

１）コンパイルエラーを収集する。
２）コンパイルエラーをなくす方法を検討する。
コンパイルエラーになる例を示すだけが目的のコードは、コンパイルエラーをなくすのではなく、コンパイルエラーの種類を収集するだけにする。
文法を示すのが目的のコード場合に、コンパイルエラーをなくすのに手間がかかる場合は、順次作業します。
３）リンクエラーをなくす方法を検討する。
文法を示すのが目的のコード場合に、リンクエラーをなくすのに手間がかかる場合は、順次作業します。
４）意味のある出力を作る。
コンパイル、リンクが通っても、意味のある出力を示そうとすると、コンパイル・リンクエラーが出て収拾できそうにない場合がある。順次作業します。

１）だけのものから４）まで進んだものと色々ある状態です。一歩でも前に進むご助言をお待ちしています。「検討事項」の欄に現状を記録するようにしています。

C++N4910:2022 Standard Working Draft on ISO/IEC 14882(0) sample code compile list

C++N4741, 2018 Standard Working Draft on ISO/IEC 14882 sample code compile list

C++N4606, 2016符号断片編纂一覧(example code compile list)

C++N4606, 2016 Working Draft 2016, ISO/IEC 14882, C++ standard(1) Example code compile list
https://qiita.com/kaizen_nagoya/items/df5d62c35bd6ed1c3d43/

C++N3242, 2011 sample code compile list on clang++ and g++

編纂器(Compiler)

clang++ --version

Debian clang version 14.0.5-++20220610033153+c12386ae247c-1~exp1~20220610153237.151
Target: x86_64-pc-linux-gnu, Thread model: posix, InstalledDir: /usr/bin

g++- --version

g++ (GCC) 12.1.0 Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

22.2.4 Forward/move helpers [forward] C++N4910:2022 (523) p666b.cpp

算譜(source code)

p666b.cpp
// C++N4910 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf
const char * n4910 = "22.2.4 Forward/move helpers [forward] C++N4910:2022 (523) p666b.cpp";
// Debian clang version 14.0.5-++20220610033153+c12386ae247c-
// g++ (GCC) 12.1.0 Copyright (C) 2022 Free Software Foundation, Inc.
// Edited by Dr. OGAWA Kiyoshi. Compile procedure and results record.
// C++N4910:2022 Standard Working Draft on ISO/IEC 14882(0) sample code compile list
// https://qiita.com/kaizen_nagoya/items/fc957ddddd402004bb91

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <coroutine>
#include <vector>
#include <complex>
#include <map>
#include <atomic>
#include <unordered_map>
#include <memory>
#include <typeinfo>

using namespace std;

//
template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept;
template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
//  Mandates: For the second overload, is_lvalue_reference_v<T> is false. //  //  Returns: static_cast<T&&>(t).
// [Example 1:
template<class T, class A1, class A2>
shared_ptr<T> factory(A1&& a1, A2&& a2) {
    return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
}
struct A {
    A(int&, const double&);
};
void g() {
    shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
    shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
}
//  In the first call to factory, A1 is deduced as int, so 2 is forwarded to A’s constructor as an rvalue. In the second call to factory, A1 is deduced as int&, so i is forwarded to A’s constructor as an lvalue. In both cases, A2 is deduced as double, so 1.414 is forwarded to A’s constructor as an rvalue.
template<class T> constexpr remove_reference_t<T>&& move(T&& t) noexcept;
// Returns: static_cast<remove_reference_t<T>&&>(t). [Example 2:
template<class T, class A1>
shared_ptr<T> factory(A1&& a1) {
    return shared_ptr<T>(new T(std::forward<A1>(a1)));
}
//  Returns: std::move(x).
struct A {
    A();
    A(const A&);
    A(A&&);
    void g() {
        A a;
        shared_ptr<A> sp1 = factory<A>(a);
        shared_ptr<A> sp2 = factory<A>(std::move(a));
// copies from lvalues // moves from rvalues
    };
}
//  In the first call to factory, A1 is deduced as A&, so a is forwarded as a non-const lvalue. This binds to the constructor A(const A&), which copies the value from a. In the second call to factory, because of the call std::move(a), A1 is deduced as A, so a is forwarded as an rvalue. This binds to the constructor A(A&&), which moves the value from a.
template<class T> constexpr conditional_t<
!is_nothrow_move_constructible_v<T> && is_copy_constructible_v<T>, const T&, T&&>
move_if_noexcept(T& x) noexcept;
int main() {
    cout  <<  n4910 << endl;
    return EXIT_SUCCESS;
}

編纂・実行結果(compile and go)

bash
$ clang++ p666b.cpp -std=03 -o p666bl -I. -Wall
In file included from p666b.cpp:19:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/atomic:38:
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/c++0x_warning.h:32:2: error: This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
#error This file requires compiler and library support \
 ^
p666b.cpp:27:20: error: unknown type name 'constexpr'
 template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept;
                   ^
p666b.cpp:27:30: warning: variable templates are a C++14 extension [-Wc++14-extensions]
 template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept;
                             ^
p666b.cpp:27:31: error: expected ';' at end of declaration
 template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept;
                              ^
                              ;
p666b.cpp:27:31: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
p666b.cpp:27:68: error: expected function body after function declarator
 template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept;
                                                                   ^
p666b.cpp:28:22: error: unknown type name 'constexpr'
   template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
                     ^
p666b.cpp:28:32: warning: variable templates are a C++14 extension [-Wc++14-extensions]
   template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
                               ^
p666b.cpp:28:33: error: expected ';' at end of declaration
   template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
                                ^
                                ;
p666b.cpp:28:33: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
p666b.cpp:28:65: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
   template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
                                                                ^
p666b.cpp:28:71: error: expected function body after function declarator
   template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
                                                                      ^
p666b.cpp:32:8: error: no template named 'shared_ptr'
       shared_ptr<T> factory(A1&& a1, A2&& a2) {
       ^
p666b.cpp:32:32: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
       shared_ptr<T> factory(A1&& a1, A2&& a2) {
                               ^
p666b.cpp:32:41: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
       shared_ptr<T> factory(A1&& a1, A2&& a2) {
                                        ^
p666b.cpp:33:42: error: no member named 'forward' in namespace 'std'
         return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
                                    ~~~~~^
p666b.cpp:33:50: error: 'A1' does not refer to a value
         return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
                                                 ^
p666b.cpp:31:32: note: declared here
       template<class T, class A1, class A2>
                               ^
p666b.cpp:33:64: error: no member named 'forward' in namespace 'std'
         return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
                                                          ~~~~~^
p666b.cpp:33:72: error: 'A2' does not refer to a value
         return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
                                                                       ^
p666b.cpp:31:42: note: declared here
       template<class T, class A1, class A2>
                                         ^
p666b.cpp:39:1: error: no template named 'shared_ptr'
shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
^
p666b.cpp:39:21: error: no matching function for call to 'factory'
shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
                    ^~~~~~~~~~
p666b.cpp:40:15: error: use of undeclared identifier 'sp2'; did you mean 'sp1'?
shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
              ^~~
              sp1
p666b.cpp:39:15: note: 'sp1' declared here
shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
              ^
p666b.cpp:40:14: error: expected ';' after expression
shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
             ^
             ;
p666b.cpp:40:1: error: use of undeclared identifier 'shared_ptr'
shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
^
p666b.cpp:40:32: error: use of undeclared identifier 'i'
shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
                               ^
p666b.cpp:43:19: error: unknown type name 'constexpr'
template<class T> constexpr remove_reference_t<T>&& move(T&& t) noexcept;
                  ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
7 warnings and 20 errors generated.
$ clang++ p666b.cpp -std=2b -o p666bl -I. -Wall
p666b.cpp:40:32: error: use of undeclared identifier 'i'
shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
                               ^
p666b.cpp:50:8: error: redefinition of 'A'
struct A {
       ^
p666b.cpp:35:15: note: previous definition is here
       struct A {
              ^
p666b.cpp:59:2: error: expected ';' after struct
}
 ^
 ;
p666b.cpp:33:35: error: no matching constructor for initialization of 'A'
         return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
                                  ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
p666b.cpp:39:21: note: in instantiation of function template specialization 'factory<A, int, double>' requested here
shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
                    ^
p666b.cpp:36:10: note: candidate constructor not viable: expects an lvalue for 1st argument
         A(int&, const double&);
         ^
p666b.cpp:35:15: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 2 were provided
       struct A {
              ^
p666b.cpp:35:15: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 2 were provided
4 errors generated.

$ g++ p666b.cpp -std=03 -o p666bg -I. -Wall
In file included from /usr/local/include/c++/12.1.0/atomic:38,
                 from p666b.cpp:19:
/usr/local/include/c++/12.1.0/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
   32 | #error This file requires compiler and library support \
      |  ^~~~~
p666b.cpp:27:20: warning: identifier 'constexpr' is a keyword in C++11 [-Wc++11-compat]
   27 |  template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept;
      |                    ^~~~~~~~~
p666b.cpp:27:68: warning: identifier 'noexcept' is a keyword in C++11 [-Wc++11-compat]
   27 |  template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept;
      |                                                                    ^~~~~~~~
p666b.cpp:27:20: error: 'constexpr' does not name a type
   27 |  template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept;
      |                    ^~~~~~~~~
p666b.cpp:27:20: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p666b.cpp:28:22: error: 'constexpr' does not name a type
   28 |    template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
      |                      ^~~~~~~~~
p666b.cpp:28:22: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p666b.cpp:32:8: error: 'shared_ptr' does not name a type
   32 |        shared_ptr<T> factory(A1&& a1, A2&& a2) {
      |        ^~~~~~~~~~
p666b.cpp: In function 'void g()':
p666b.cpp:39:1: error: 'shared_ptr' was not declared in this scope
   39 | shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
      | ^~~~~~~~~~
p666b.cpp:39:1: note: 'std::shared_ptr' is only available from C++11 onwards
p666b.cpp:39:13: error: expected primary-expression before '>' token
   39 | shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
      |             ^
p666b.cpp:39:15: error: 'sp1' was not declared in this scope
   39 | shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
      |               ^~~
p666b.cpp:39:21: error: 'factory' was not declared in this scope
   39 | shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
      |                     ^~~~~~~
p666b.cpp:39:30: error: expected primary-expression before '>' token
   39 | shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
      |                              ^
p666b.cpp:39:32: warning: left operand of comma operator has no effect [-Wunused-value]
   39 | shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int& int i = 2;
      |                                ^
p666b.cpp:40:13: error: expected primary-expression before '>' token
   40 | shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
      |             ^
p666b.cpp:40:15: error: 'sp2' was not declared in this scope
   40 | shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
      |               ^~~
p666b.cpp:40:30: error: expected primary-expression before '>' token
   40 | shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
      |                              ^
p666b.cpp:40:32: error: 'i' was not declared in this scope
   40 | shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
      |                                ^
p666b.cpp: At global scope:
p666b.cpp:43:19: error: 'constexpr' does not name a type
   43 | template<class T> constexpr remove_reference_t<T>&& move(T&& t) noexcept;
      |                   ^~~~~~~~~
p666b.cpp:43:19: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p666b.cpp:46:8: error: 'shared_ptr' does not name a type
   46 |        shared_ptr<T> factory(A1&& a1) {
      |        ^~~~~~~~~~
p666b.cpp:50:8: error: redefinition of 'struct A'
   50 | struct A {
      |        ^
p666b.cpp:35:15: note: previous definition of 'struct A'
   35 |        struct A {
      |               ^
p666b.cpp:61:1: error: expected unqualified-id before 'template'
   61 | template<class T> constexpr conditional_t<
      | ^~~~~~~~

$ g++ p666b.cpp -std=2b -o p666bg -I. -Wall
p666b.cpp: In function 'void g()':
p666b.cpp:40:32: error: 'i' was not declared in this scope
   40 | shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
      |                                ^
p666b.cpp: At global scope:
p666b.cpp:50:8: error: redefinition of 'struct A'
   50 | struct A {
      |        ^
p666b.cpp:35:15: note: previous definition of 'struct A'
   35 |        struct A {
      |               ^
p666b.cpp:61:1: error: expected unqualified-id before 'template'
   61 | template<class T> constexpr conditional_t<
      | ^~~~~~~~
p666b.cpp: In instantiation of 'std::shared_ptr<_Tp> factory(A1&&, A2&&) [with T = A; A1 = int; A2 = double]':
p666b.cpp:39:31:   required from here
p666b.cpp:33:31: error: cannot bind non-const lvalue reference of type 'int&' to an rvalue of type 'int'
   33 |          return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
      |                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
p666b.cpp:36:12: note:   initializing argument 1 of 'A::A(int&, const double&)'
   36 |          A(int&, const double&);
      |            ^~~~

検討事項(agenda)

コンパイルエラーを取るか、コンパイルエラーの理由を解説する。

参考資料(reference)

cpprefjp - C++日本語リファレンス

コンパイラの実装状況

typedef は C++11 ではオワコン

C99からC++14を駆け抜けるC++講座

@kazuo_reve 私が効果を確認した「小川メソッド」

自己参照(self reference)

ockerにclang

docker gnu(gcc/g++) and llvm(clang/clang++)

コンパイル用shell script C版(clangとgcc)とC++版(clang++とg++)

C++N4910:2022 tag follower 200人超えました。ありがとうございます。

C++N4910:2022 tag follower 200人超えました。ありがとうございます。

コピペコンパイルエラーあるある

C++ Error Message Collection(1)does not name a type, 11 articles

dockerにclang

docker gnu(gcc/g++) and llvm(clang/clang++)

コンパイル用shell script C版(clangとgcc)とC++版(clang++とg++)

Compare the contents of C++N4910:2022, C++N4741:2018 and C++N4606:2015

C++ sample list

clang++, g++コンパイルエラー方針の違いの例

astyle 使ってみた

C++N4606 Working Draft 2016, ISO/IEC 14882, C++ standardのコード断片をコンパイルするためにしていること
https://qiita.com/kaizen_nagoya/items/a8d7ee2f2e29e76c19c1

コンパイル用shell script C版(clangとgcc)とC++版(clang++とg++)
https://qiita.com/kaizen_nagoya/items/74220c0577a512c2d7da

Clang/Clang++(LLVM) gcc/g++(GNU) コンパイラ警告等比較
https://qiita.com/kaizen_nagoya/items/9a82b958cc3aeef0403f

C++2003とC++2017でコンパイルエラーになるならない事例集
https://qiita.com/kaizen_nagoya/items/a13ea3823441c430edff

Qiitaに投稿するCのStyle例(暫定)
https://qiita.com/kaizen_nagoya/items/946df1528a6a1ef2bc0d

cpprefjpのdecltypeをコンパイル試験
https://qiita.com/kaizen_nagoya/items/090909af702f0d5d8a67

MISRA C++ 5-0-16
https://qiita.com/kaizen_nagoya/items/7df2d4e05db724752a74

C++ Templates Part1 BASICS Chapter 3. Class Templates 3.2 Use of Class Template Stack stack1test.cpp
https://qiita.com/kaizen_nagoya/items/cd5fc49106fad5a4e9ed

ISO/IEC TS 17961:2013 C Secure Coding Rules(1) All list(to be confirmed)
https://qiita.com/kaizen_nagoya/items/54e056195c4f11b850a1

C言語（C++)に対する誤解、曲解、無理解、爽快。
https://qiita.com/kaizen_nagoya/items/3f3992c9722c1cee2e3a

C Puzzle Bookの有り難み5つ、C言語規格及びCコンパイラの特性を認識
https://qiita.com/kaizen_nagoya/items/d89a48c1536a02ecdec9

'wchar.h' file not found で困った clang++ macOS
https://qiita.com/kaizen_nagoya/items/de15cd46d657517fac11

Open POSIX Test Suiteの使い方を調べはじめました
https://qiita.com/kaizen_nagoya/items/644d5e407f5faf96e6dc

MISRA-C 2012 Referenceに掲載している文献の入手可能性を確認
https://qiita.com/kaizen_nagoya/items/96dc8b125e462d5575bb

どうやって MISRA Example Suiteをコンパイルするか
https://qiita.com/kaizen_nagoya/items/fbdbff5ff696e2ca7f00

MISRA C まとめ #include
https://qiita.com/kaizen_nagoya/items/f1a79a7cbd281607c7c9

「C++完全理解ガイド」の同意できること上位10
https://qiita.com/kaizen_nagoya/items/aa5744e0c4a8618c7671

DoCAP（ドゥーキャップ）って何ですか？

小川メソッド　覚え（書きかけ）

＜この記事は個人の過去の経験に基づく個人の感想です。現在所属する組織、業務とは関係がありません。＞

文書履歴(document history)

ver. 0.01 初稿 　20220723

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