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@kaizen_nagoya(Dr. Kiyoshi Ogawa)

C++N4741(113)11.5 Structured binding declarations [dcl.struct.bind] p196

Last updated at Posted at 2018-05-25

#はじめに(Introduction)
C++N4741 Working Draft, Standard for Programming Language C++
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4741.pdf

C++N4741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。

##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4つの情報に基づいています。

https://stackoverflow.com
https://cpprefjp.github.io
http://ja.cppreference.com/
https://teratail.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++17, C++2aの３種類
g++では-std=c++03, c++17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。

初めての　CEDD(Compile Error Driven Design) ８回直してコンパイル。
https://qiita.com/kaizen_nagoya/items/9494236aa1753f3fd1e1

##C++N4741, 2018 Standard Working Draft on ISO/IEC 14882 sample code compile list
https://qiita.com/kaizen_nagoya/items/3294c014044550896010

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++
https://qiita.com/kaizen_nagoya/items/685b5c1a2c17c1bf1318

編纂器(Compiler)

###clang++ --version
clang version 6.0.0 (tags/RELEASE_600/final)
Target: x86_64-apple-darwin17.4.0

###g++-7 --version
g++-7 (Homebrew GCC 7.3.0_1) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.

#(113)11.5 Structured binding declarations [dcl.struct.bind] p196

C++N4606(102) 8.5 Decomposition declarations [dcl.decomp] p219
https://qiita.com/kaizen_nagoya/items/7de66746ded630f6950c

No examples at C++N3242, 2011

##p196.cpp
###算譜(source code)

p196.cpp
// C++N4741 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n741.pdf
const char* msg= "C++N4741(113)11.5 Structured binding declarations [dcl.struct.bind] p196.cpp";
// Edited by Dr. Ogawa Kiyoshi. Compile procedure and results record.

#include <iostream>
#include <cstdlib>

using namespace std;

auto f() -> int(&)[2] {};
auto [ x, y ] = f(); // x and y refer to elements in a copy of the array return value
auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array referred to by f’s return value

namespace M {
  struct S {
    int x1 : 2;
    volatile double y1;
  };
  S f() {};
  const auto [ x, y ] = f();
}

int main() {
  cout<< msg << endl;
  return EXIT_SUCCESS;
}

###編纂・実行結果(compile and go)

cpla.sh
$ ../cpla.sh p196
$ clang++ p196.cpp -std=c++03 -Wall
p196.cpp:11:1: warning: 'auto' type specifier is a C++11 extension
      [-Wc++11-extensions]
auto f() -> int(&)[2]{};
^
p196.cpp:11:10: error: expected function body after function declarator
auto f() -> int(&)[2]{};
         ^
p196.cpp:12:1: warning: 'auto' type specifier is a C++11 extension
      [-Wc++11-extensions]
auto [ x, y ] = f(); // x and y refer to elements in a copy of the arra...
^
p196.cpp:12:6: warning: decomposition declarations are a C++17 extension
      [-Wc++17-extensions]
auto [ x, y ] = f(); // x and y refer to elements in a copy of the arra...
     ^~~~~~~~
p196.cpp:12:17: error: use of undeclared identifier 'f'
auto [ x, y ] = f(); // x and y refer to elements in a copy of the arra...
                ^
p196.cpp:13:1: warning: 'auto' type specifier is a C++11 extension
      [-Wc++11-extensions]
auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array refe...
^
p196.cpp:13:7: warning: decomposition declarations are a C++17 extension
      [-Wc++17-extensions]
auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array refe...
      ^~~~~~~~~~
p196.cpp:13:20: error: use of undeclared identifier 'f'
auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array refe...
                   ^
p196.cpp:18:7: warning: 'auto' type specifier is a C++11 extension
      [-Wc++11-extensions]
const auto [ x, y ] = f();
      ^
p196.cpp:18:12: warning: decomposition declarations are a C++17 extension
      [-Wc++17-extensions]
const auto [ x, y ] = f();
           ^~~~~~~~
In file included from p196.cpp:6:
In file included from /usr/local/Cellar/llvm/6.0.0/include/c++/v1/iostream:38:
In file included from /usr/local/Cellar/llvm/6.0.0/include/c++/v1/ios:216:
In file included from /usr/local/Cellar/llvm/6.0.0/include/c++/v1/__locale:15:
In file included from /usr/local/Cellar/llvm/6.0.0/include/c++/v1/string:477:
In file included from /usr/local/Cellar/llvm/6.0.0/include/c++/v1/string_view:176:
In file included from /usr/local/Cellar/llvm/6.0.0/include/c++/v1/__string:56:
In file included from /usr/local/Cellar/llvm/6.0.0/include/c++/v1/algorithm:642:
In file included from /usr/local/Cellar/llvm/6.0.0/include/c++/v1/utility:198:
/usr/local/Cellar/llvm/6.0.0/include/c++/v1/__tuple:52:80: error: implicit
      instantiation of undefined template 'std::__1::tuple_size<M::S>'
  ..._Tp> class _LIBCPP_TEMPLATE_VIS tuple_size<const _Tp> : public tuple_siz...
                                                                    ^
p196.cpp:18:12: note: in instantiation of template class
      'std::__1::tuple_size<const M::S>' requested here
const auto [ x, y ] = f();
           ^
/usr/local/Cellar/llvm/6.0.0/include/c++/v1/__tuple:25:49: note: template is
      declared here
template <class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_size;
                                                ^
p196.cpp:18:12: error: cannot decompose this type; 'std::tuple_size<const
      M::S>::value' is not a valid integral constant expression
const auto [ x, y ] = f();
           ^
7 warnings and 5 errors generated.
$ clang++ p196.cpp -std=c++2a -Wall
p196.cpp:11:23: warning: control reaches end of non-void function
      [-Wreturn-type]
auto f() -> int(&)[2]{};
                      ^
p196.cpp:17:7: warning: control reaches end of non-void function [-Wreturn-type]
S f(){};
      ^
2 warnings generated.
../cpla.sh: line 11: 35463 Illegal instruction: 4  ./$1l2a $2

$ g++-8 p196.cpp -std=c++03  -Wall
p196.cpp:11:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
 auto f() -> int(&)[2]{};
 ^~~~
 ----
p196.cpp:11:21: error: ISO C++ forbids declaration of 'f' with no type [-fpermissive]
 auto f() -> int(&)[2]{};
                     ^
p196.cpp:11:21: error: top-level declaration of 'f' specifies 'auto'
p196.cpp:11:21: error: trailing return type only available with -std=c++11 or -std=gnu++11
p196.cpp:12:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
 auto [ x, y ] = f(); // x and y refer to elements in a copy of the array return value
 ^~~~
 ----
p196.cpp:12:6: warning: structured bindings only available with -std=c++17 or -std=gnu++17
 auto [ x, y ] = f(); // x and y refer to elements in a copy of the array return value
      ^
p196.cpp:12:13: error: ISO C++ forbids declaration of 'structured binding' with no type [-fpermissive]
 auto [ x, y ] = f(); // x and y refer to elements in a copy of the array return value
             ^
p196.cpp:12:6: error: structured binding declaration cannot be C++98 'auto'
 auto [ x, y ] = f(); // x and y refer to elements in a copy of the array return value
      ^~~~~~~~
p196.cpp:12:6: error: structured binding declaration cannot have type 'int'
p196.cpp:12:6: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'
p196.cpp:12:17: error: 'f' was not declared in this scope
 auto [ x, y ] = f(); // x and y refer to elements in a copy of the array return value
                 ^
p196.cpp:13:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
 auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array referred to by f’s return value
 ^~~~
 ----
p196.cpp:13:7: warning: structured bindings only available with -std=c++17 or -std=gnu++17
 auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array referred to by f’s return value
       ^
p196.cpp:13:16: error: ISO C++ forbids declaration of 'structured binding' with no type [-fpermissive]
 auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array referred to by f’s return value
                ^
p196.cpp:13:7: error: structured binding declaration cannot be C++98 'auto'
 auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array referred to by f’s return value
       ^~~~~~~~~~
p196.cpp:13:7: error: structured binding declaration cannot have type 'int'
p196.cpp:13:7: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'
p196.cpp:13:20: error: 'f' was not declared in this scope
 auto& [ xr, yr ] = f(); // xr and yr refer to elements in the array referred to by f’s return value
                    ^
p196.cpp: In function 'M::S M::f()':
p196.cpp:17:7: warning: no return statement in function returning non-void [-Wreturn-type]
 S f(){};
       ^
p196.cpp: At global scope:
p196.cpp:18:7: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
 const auto [ x, y ] = f();
       ^~~~
       ----
p196.cpp:18:12: warning: structured bindings only available with -std=c++17 or -std=gnu++17
 const auto [ x, y ] = f();
            ^
p196.cpp:18:19: error: ISO C++ forbids declaration of 'structured binding' with no type [-fpermissive]
 const auto [ x, y ] = f();
                   ^
p196.cpp:18:12: error: structured binding declaration cannot be C++98 'auto'
 const auto [ x, y ] = f();
            ^~~~~~~~
p196.cpp:18:12: error: structured binding declaration cannot have type 'const int'
p196.cpp:18:12: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'

$ g++-8 p196.cpp -std=c++2a  -Wall
p196.cpp: In function 'int (& f())[2]':
p196.cpp:11:23: warning: no return statement in function returning non-void [-Wreturn-type]
 auto f() -> int(&)[2]{};
                       ^
p196.cpp: In function 'M::S M::f()':
p196.cpp:17:7: warning: no return statement in function returning non-void [-Wreturn-type]
 S f(){};
       ^
../cpla.sh: line 23: 35474 Segmentation fault: 11  ./$1g2a $2

#検討事項(agenda)
実行時エラーを修正する方法。
clang++(illegal instruction ), g++(segmentation fault)で実行時エラーの種類が違う。
役に立つまたは意味のあるその他の出力

#参考資料(reference)

docker gnu(gcc/g++) and llvm(clang/clang++)
https://qiita.com/drafts/059874ea39c4de64c0f7

[C][C++]の国際規格案の例題をコンパイルするときの課題７つ。
https://qiita.com/kaizen_nagoya/items/5f4b155030259497c4de

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

#文書履歴(document history)
ver. 0.10 初稿 20180525
ver. 0.11 章番号・頁番号訂正 20180526

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