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はじめに

N4606 Working Draft, Standard for Programming Language C++
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/#mailing2016-11
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf

n4606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。

list

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

Compiler

###clang++ --version
clang version 6.0.0 (tags/RELEASE_600/final)

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

(68)7.1.5 The constexpr specifier [dcl.constexpr]
p162

p162.cpp
// N4606 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf 
#define msg "p162.cpp(68)7.1.5 The constexpr specifier [dcl.constexpr]"

#include <iostream>

struct Length {
  constexpr explicit Length(int i = 0) : val(i) { }
  private:
  int val;
};
constexpr int f(bool b)
{ return b ? throw 0 : 0; } // OK
constexpr int f() { return f(true); } // ill-formed, no diagnostic required
struct B {
  constexpr B(int x) : i(0) { } // x is unused
  int i;
};

int global;
struct D : B {
  constexpr D() : B(global) { } // ill-formed, no diagnostic required
  // lvalue-to-rvalue conversion on non-constant global
};
constexpr int bar(int x, int y) // OK
{ return x + y + x*y; }
// ...
int bar(int x, int y) // error: redefinition of bar
{ return x * 2 + 3 * y; }
  struct pixel {
  int x, y;
};
constexpr pixel ur = { 1294, 1024 };// OK
constexpr pixel origin; // error: initializer missing

int main(){
  std::cout<< msg << std::endl;
  return EXIT_SUCCESS;
}
$ ./cppgl17.sh p162
$ clang++ p162.cpp
p162.cpp:22:13: error: constexpr constructor never produces a constant expression [-Winvalid-constexpr]
  constexpr D() : B(global) { } // ill-formed, no diagnostic required
            ^
p162.cpp:22:21: note: read of non-const variable 'global' is not allowed in a constant expression
  constexpr D() : B(global) { } // ill-formed, no diagnostic required
                    ^
p162.cpp:20:5: note: declared here
int global;
    ^
p162.cpp:28:5: error: non-constexpr declaration of 'bar' follows constexpr declaration
int bar(int x, int y) // error: redefinition of bar
    ^
p162.cpp:25:15: note: previous declaration is here
constexpr int bar(int x, int y) // OK
              ^
p162.cpp:34:17: error: default initialization of an object of const type 'const pixel' without a user-provided default constructor
constexpr pixel origin; // error: initializer missing
                ^
                      {}
3 errors generated.

$ g++-7 p162.cpp
p162.cpp: In constructor 'constexpr D::D()':
p162.cpp:22:31: error: the value of 'global' is not usable in a constant expression
   constexpr D() : B(global) { } // ill-formed, no diagnostic required
                               ^
p162.cpp:20:5: note: 'int global' is not const
 int global;
     ^~~~~~
p162.cpp: In function 'int bar(int, int)':
p162.cpp:28:5: error: redefinition of 'int bar(int, int)'
 int bar(int x, int y) // error: redefinition of bar
     ^~~
p162.cpp:25:15: note: 'constexpr int bar(int, int)' previously defined here
 constexpr int bar(int x, int y) // OK
               ^~~
p162.cpp: At global scope:
p162.cpp:34:17: error: uninitialized const 'origin' [-fpermissive]
 constexpr pixel origin; // error: initializer missing
                 ^~~~~~
p162.cpp:30:10: note: 'const struct pixel' has no user-provided default constructor
   struct pixel {
          ^~~~~
p162.cpp:31:7: note: and the implicitly-defined constructor does not initialize 'int pixel::x'
   int x, y;
       ^

#検討事項
コンパイルエラーが出なくなる修正方法。
clang++とg++のコンパイルエラーの違いの背景。

#参考資料

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

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

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

文書履歴

0.10 初稿 2080417

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