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@kaizen_nagoya(Dr. Kiyoshi Ogawa)

C++ N4606 (72)7.1.7.4 The auto specifier [dcl.spec.auto] p170

Last updated at Posted at 2018-04-17

#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。

#作業方針
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。

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.

#(72)7.1.7.4 The auto specifier [dcl.spec.auto]
p170

p170.cpp
// N4606 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf 
#define msg "p170.cpp(72)7.1.7.4 The auto specifier [dcl.spec.auto]"

#include <iostream>

auto glambda = { return i; }; // OK: a generic lambda
auto x = 5; // OK: x has type int
const auto v = &x, u = 6; // OK: v has type const int, u has type const int
static auto y = 0.0; // OK: y has type double
auto int r; // error: auto is not a storage-class-specifier
auto f() -> int; // OK: f returns int
auto g() { return 0.0; } // OK: g returns double
auto h(); // OK: h’s return type will be deduced when it is defined
auto x = 5, *y = &x; // OK: auto is int
auto a = 5, b = { 1, 2 }; // error: different types for auto
auto f() { } // OK, return type is void
auto* g() { } // error, cannot deduce auto* from void()
auto n = n; // error, n’s type is unknown
auto f();
void g() { &f; } // error, f’s return type is unknown

auto sum(int i) {
  if (i == 1)
  return i; // sum’s return type is int
else
  return sum(i-1)+i; // OK, sum’s return type has been deduced
}

template auto f(T t) { return t; } // return type deduced at instantiation time
typedef decltype(f(1)) fint_t; // instantiates f to deduce return type
template auto f(T* t) { return t; }
void g() { int (*p)(int) = &f; } // instantiates both fs to determine return types,
// chooses second
auto f();
auto f() { return 42; } // return type is int
auto f(); // OK
int f(); // error, cannot be overloaded with auto f()
decltype(auto) f(); // error, auto and decltype(auto) don’t match
template auto g(T t) { return t; } // #1
template auto g(int); // OK, return type is int
template char g(char); // error, no matching template
template<> auto g(double); // OK, forward declaration with unknown return type
template T g(T t) { return t; } // OK, not functionally equivalent to #1
template char g(char); // OK, now there is a matching template
template auto g(float); // still matches #1
void h() { return g(42); } // error, ambiguous
template struct A {
  friend T frf(T);
};
auto frf(int i) { return i; } // not a friend of A
template auto f(T t) { return t; }
extern template auto f(int); // does not instantiate f
int (*p)(int) = f; // instantiates f to determine its return type, but an explicit
// instantiation definition is still required somewhere in the program

int main(){
  int i=1;
  foo(i);
  std::cout<< msg << std::endl;
  return EXIT_SUCCESS;
}

$ ./cppgl17.sh p172
$ clang++ p172.cpp
p172.cpp:7:18: error: expected expression
auto glambda = { return i; }; // OK: a generic lambda
                 ^
p172.cpp:9:7: error: 'auto' deduced as 'int *' in declaration of 'v' and deduced as 'int' in declaration of 'u'
const auto v = &x, u = 6; // OK: v has type const int, u has type const int
      ^        ~~      ~
p172.cpp:11:1: warning: 'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases
      [-Wauto-storage-class]
auto int r; // error: auto is not a storage-class-specifier
^~~~~
p172.cpp:11:10: error: illegal storage class on file-scoped variable
auto int r; // error: auto is not a storage-class-specifier
         ^
p172.cpp:15:6: error: redefinition of 'x'
auto x = 5, *y = &x; // OK: auto is int
     ^
p172.cpp:8:6: note: previous definition is here
auto x = 5; // OK: x has type int
     ^
p172.cpp:15:14: error: non-static declaration of 'y' follows static declaration
auto x = 5, *y = &x; // OK: auto is int
             ^
p172.cpp:10:13: note: previous definition is here
static auto y = 0.0; // OK: y has type double
            ^
p172.cpp:16:1: error: 'auto' deduced as 'int' in declaration of 'a' and deduced as 'std::initializer_list<int>' in declaration of 'b'
auto a = 5, b = { 1, 2 }; // error: different types for auto
^        ~      ~~~~~~~~
p172.cpp:17:6: error: functions that differ only in their return type cannot be overloaded
auto f() { } // OK, return type is void
~~~~ ^
p172.cpp:12:6: note: previous declaration is here
auto f() -> int; // OK: f returns int
     ^
p172.cpp:18:7: error: functions that differ only in their return type cannot be overloaded
auto* g() { } // error, cannot deduce auto* from void()
~~~~~ ^
p172.cpp:13:6: note: previous definition is here
auto g() { return 0.0; } // OK: g returns double
~~~~ ^
p172.cpp:19:10: error: variable 'n' declared with deduced type 'auto' cannot appear in its own initializer
auto n = n; // error, n’s type is unknown
         ^
p172.cpp:20:6: error: functions that differ only in their return type cannot be overloaded
auto f();
~~~~ ^
p172.cpp:12:6: note: previous declaration is here
auto f() -> int; // OK: f returns int
     ^
p172.cpp:21:6: error: functions that differ only in their return type cannot be overloaded
void g() { &f; } // error, f’s return type is unknown
~~~~ ^
p172.cpp:13:6: note: previous definition is here
auto g() { return 0.0; } // OK: g returns double
~~~~ ^
p172.cpp:21:12: warning: expression result unused [-Wunused-value]
void g() { &f; } // error, f’s return type is unknown
           ^~
p172.cpp:30:17: error: unknown type name 'T'
template auto f(T t) { return t; } // return type deduced at instantiation time
                ^
p172.cpp:30:22: error: function cannot be defined in an explicit instantiation; if this declaration is meant to be a function definition,
      remove the 'template' keyword
template auto f(T t) { return t; } // return type deduced at instantiation time
                     ^
p172.cpp:31:18: error: no matching function for call to 'f'
typedef decltype(f(1)) fint_t; // instantiates f to deduce return type
                 ^
p172.cpp:12:6: note: candidate function not viable: requires 0 arguments, but 1 was provided
auto f() -> int; // OK: f returns int
     ^
p172.cpp:32:17: error: unknown type name 'T'
template auto f(T* t) { return t; }
                ^
p172.cpp:32:23: error: function cannot be defined in an explicit instantiation; if this declaration is meant to be a function definition,
      remove the 'template' keyword
template auto f(T* t) { return t; }
                      ^
p172.cpp:33:6: error: functions that differ only in their return type cannot be overloaded
void g() { int (*p)(int) = &f; } // instantiates both fs to determine return types,
~~~~ ^
p172.cpp:13:6: note: previous definition is here
auto g() { return 0.0; } // OK: g returns double
~~~~ ^
p172.cpp:33:18: error: cannot initialize a variable of type 'int (*)(int)' with an rvalue of type 'auto (*)() -> int': different number of
      parameters (1 vs 0)
void g() { int (*p)(int) = &f; } // instantiates both fs to determine return types,
                 ^         ~~
p172.cpp:35:6: error: functions that differ only in their return type cannot be overloaded
auto f();
~~~~ ^
p172.cpp:12:6: note: previous declaration is here
auto f() -> int; // OK: f returns int
     ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
2 warnings and 20 errors generated.

$ g++-7 p172.cpp
p172.cpp:7:18: error: expected primary-expression before 'return'
 auto glambda = { return i; }; // OK: a generic lambda
                  ^~~~~~
p172.cpp:7:18: error: expected '}' before 'return'
p172.cpp:7:18: error: unable to deduce 'std::initializer_list<auto>' from '{<expression error>}'
p172.cpp:7:18: error: expected ',' or ';' before 'return'
p172.cpp:7:28: error: expected declaration before '}' token
 auto glambda = { return i; }; // OK: a generic lambda
                            ^

#検討事項
clang++とg++の実行時エラーの有無の背景。
clang++ 実行中断(libc++abi.dylib: __cxa_guard_acquire detected deadlock, Abort trap: 6)
g++ 無限ループ？(^Cで手動終了)

#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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