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@kaizen_nagoya

N4606 Working Draft 2016, ISO/IEC 14882, C++ standard(30)4.1 Lvalue-to-rvalue conversion [conv.lval] p85

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擦れの確認が結果としてできれば幸いです。

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.

4 Standard conversions [conv]

(30)4.1 Lvalue-to-rvalue conversion [conv.lval]

p85

p85.cpp
// N4606 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf 
#define msg "p85.cpp (30)4.1 Lvalue-to-rvalue conversion [conv.lval]"

#include <iostream>

struct S { int n; };
auto f() {
  S x { 1 };
  constexpr S y { 2 };
  return [&](bool b) { return (b ? y : x).n; };
}

int main(int argc, char *argv[], char *envp[]){

  auto g = f();
  int m = g(false); // undefined behavior due to access of x.n outside its lifetime
  int n = g(true); // OK, does not access y.n

  std::cout <<" m=" << m << " n=" << n << std::endl;
  std::cout << msg << std::endl;
  return EXIT_SUCCESS;
}
$ ./cppgl17.sh p85
$ clang++ p85.cpp
 m=-294967480 n=2
p85.cpp (30)4.1 Lvalue-to-rvalue conversion [conv.lval]

$ g++-7 p85.cpp
 m=32766 n=2
p85.cpp (30)4.1 Lvalue-to-rvalue conversion [conv.lval]

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

文書履歴

0.10 初稿 2080415

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