6.8.1 General [basic.types.general] C++N4910:2022 (28) p72.cpp

# はじめに(Introduction)

N4910  Working Draft, Standard for Programming Language C++

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf

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。

## 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  

https://qiita.com/kaizen_nagoya/items/fc957ddddd402004bb91


### 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
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.

# 6.8.1 General [basic.types.general] C++N4910:2022 (28) p72.cpp

## 算譜(source code)

```c++:p72.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 * msg="6.8.1 General [basic.types.general] C++N4910:2022 (28) p72.cpp";
// Edited by Dr. Ogawa Kiyoshi. Compile procedure and results record.

#include <iostream>
#include <cstdlib> 
#include <cstring>

using namespace std;

// [Example 1 :
constexpr std::size_t N = sizeof(T);
char buf[N];
T obj; // obj initialized to its original value
std::memcpy(buf, &obj, N); // between these two calls to std::memcpy, obj might be modified
std::memcpy(&obj, buf, N); // at this point, each subobject of obj of scalar type holds its original value
// [Example 2 :
T* t1p;
T* t2p;
// provided that t2p points to an initialized object ...
std::memcpy(t1p, t2p, sizeof(T));
// at this point, every subobject of trivially copyable type in *t1p contains
// the same value as the corresponding subobject in *t2p
// [Example 3 :
class X; // X is an incomplete type
extern X* xp; // xp is a pointer to an incomplete type
extern int arr[]; // the type of arr is incomplete
typedef int UNKA[]; // UNKA is an incomplete type
UNKA* arrp; // arrp is a pointer to an incomplete type
UNKA** arrpp;
void foo() {
xp++; // error: X is incomplete
arrp++; // error: incomplete type
arrpp++; // OK, sizeof UNKA* is known
}
struct X { int i; }; // now X is a complete type
int arr[10]; // now the type of arr is complete
X x;
void bar() {
xp = &x; // OK; type is “pointer to X”
arrp = &arr; // OK; qualification conversion (7.3.6)
xp++; // OK, X is complete
arrp++; // error: UNKA can’t be completed
}
int main() {
  cout <<  msg << endl;
  return EXIT_SUCCESS;
}
```

## Script

```bash:clgc.sh
#!/bin/sh
rm $1l
rm $1g
echo "$ clang++ $1.cpp -std=03 -o $1l -I. -Wall" 
clang++ $1.cpp -std=c++03 -o $1l -I. -Wall
if [  -e $1l ]; then
./$1l 
fi
rm $1l
echo "$ clang++ $1.cpp -std=2b -o $1l -I. -Wall"
clang++ $1.cpp -std=c++2b -o $1l -I. -Wall
if [  -e $1l ]; then
./$1l
fi
echo "\r"
echo "$ g++ $1.cpp -std=03 -o $1g -I. -Wall"
g++ $1.cpp -std=c++03 -o $1g -I. -Wall
if [ -e $1g ]; then
./$1g
fi
rm $1g
echo "\r"
echo "$ g++ $1.cpp -std=2b -o $1g -I. -Wall"
g++ $1.cpp -std=c++2b -o $1g -I. -Wall
if [ -e $1g ]; then
./$1g 
fi
```
## 編纂・実行結果(compile and go)

```ubuntu:bash
# ./clgc.sh p72
rm: cannot remove 'p72l': No such file or directory
rm: cannot remove 'p72g': No such file or directory
$ clang++ p72.cpp -std=03 -o p72l -I. -Wall
p72.cpp:13:1: error: unknown type name 'constexpr'
constexpr std::size_t N = sizeof(T);
^
p72.cpp:13:22: error: expected ';' after top level declarator
constexpr std::size_t N = sizeof(T);
                     ^
                     ;
p72.cpp:14:10: error: use of undeclared identifier 'N'
char buf[N];
         ^
p72.cpp:15:1: error: unknown type name 'T'
T obj; // obj initialized to its original value
^
p72.cpp:16:6: error: C++ requires a type specifier for all declarations
std::memcpy(buf, &obj, N); // between these two calls to std::memcpy, obj might be modified
     ^
p72.cpp:16:6: error: no member named 'memcpy' in namespace 'std'
std::memcpy(buf, &obj, N); // between these two calls to std::memcpy, obj might be modified
~~~~~^
p72.cpp:16:24: error: use of undeclared identifier 'N'
std::memcpy(buf, &obj, N); // between these two calls to std::memcpy, obj might be modified
                       ^
p72.cpp:17:6: error: C++ requires a type specifier for all declarations
std::memcpy(&obj, buf, N); // at this point, each subobject of obj of scalar type holds its original value
     ^
p72.cpp:17:6: error: no member named 'memcpy' in namespace 'std'
std::memcpy(&obj, buf, N); // at this point, each subobject of obj of scalar type holds its original value
~~~~~^
p72.cpp:17:24: error: use of undeclared identifier 'N'
std::memcpy(&obj, buf, N); // at this point, each subobject of obj of scalar type holds its original value
                       ^
p72.cpp:19:1: error: unknown type name 'T'
T* t1p;
^
p72.cpp:20:1: error: unknown type name 'T'
T* t2p;
^
p72.cpp:22:6: error: C++ requires a type specifier for all declarations
std::memcpy(t1p, t2p, sizeof(T));
     ^
p72.cpp:22:6: error: no member named 'memcpy' in namespace 'std'
std::memcpy(t1p, t2p, sizeof(T));
~~~~~^
p72.cpp:22:30: error: use of undeclared identifier 'T'
std::memcpy(t1p, t2p, sizeof(T));
                             ^
p72.cpp:33:3: error: arithmetic on a pointer to an incomplete type 'X'
xp++; // error: X is incomplete
~~^
p72.cpp:26:7: note: forward declaration of 'X'
class X; // X is an incomplete type
      ^
p72.cpp:34:5: error: arithmetic on a pointer to an incomplete type 'UNKA' (aka 'int[]')
arrp++; // error: incomplete type
~~~~^
p72.cpp:37:1: warning: 'X' defined as a struct here but previously declared as a class; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]
struct X { int i; }; // now X is a complete type
^
p72.cpp:26:1: note: did you mean struct here?
class X; // X is an incomplete type
^~~~~
struct
p72.cpp:42:8: error: incompatible pointer types assigning to 'UNKA *' (aka 'int (*)[]') from 'int (*)[10]'
arrp = &arr; // OK; qualification conversion (7.3.6)
       ^~~~
p72.cpp:44:5: error: arithmetic on a pointer to an incomplete type 'UNKA' (aka 'int[]')
arrp++; // error: UNKA can’t be completed
~~~~^
1 warning and 19 errors generated.
rm: cannot remove 'p72l': No such file or directory
$ clang++ p72.cpp -std=2b -o p72l -I. -Wall
p72.cpp:13:34: error: use of undeclared identifier 'T'
constexpr std::size_t N = sizeof(T);
                                 ^
p72.cpp:14:6: error: variable length array declaration not allowed at file scope
char buf[N];
     ^   ~
p72.cpp:15:1: error: unknown type name 'T'
T obj; // obj initialized to its original value
^
p72.cpp:16:6: error: C++ requires a type specifier for all declarations
std::memcpy(buf, &obj, N); // between these two calls to std::memcpy, obj might be modified
     ^
p72.cpp:16:6: error: no member named 'memcpy' in namespace 'std'
std::memcpy(buf, &obj, N); // between these two calls to std::memcpy, obj might be modified
~~~~~^
p72.cpp:17:6: error: C++ requires a type specifier for all declarations
std::memcpy(&obj, buf, N); // at this point, each subobject of obj of scalar type holds its original value
     ^
p72.cpp:17:6: error: no member named 'memcpy' in namespace 'std'
std::memcpy(&obj, buf, N); // at this point, each subobject of obj of scalar type holds its original value
~~~~~^
p72.cpp:19:1: error: unknown type name 'T'
T* t1p;
^
p72.cpp:20:1: error: unknown type name 'T'
T* t2p;
^
p72.cpp:22:6: error: C++ requires a type specifier for all declarations
std::memcpy(t1p, t2p, sizeof(T));
     ^
p72.cpp:22:6: error: no member named 'memcpy' in namespace 'std'
std::memcpy(t1p, t2p, sizeof(T));
~~~~~^
p72.cpp:22:30: error: use of undeclared identifier 'T'
std::memcpy(t1p, t2p, sizeof(T));
                             ^
p72.cpp:33:3: error: arithmetic on a pointer to an incomplete type 'X'
xp++; // error: X is incomplete
~~^
p72.cpp:26:7: note: forward declaration of 'X'
class X; // X is an incomplete type
      ^
p72.cpp:34:5: error: arithmetic on a pointer to an incomplete type 'UNKA' (aka 'int[]')
arrp++; // error: incomplete type
~~~~^
p72.cpp:37:1: warning: 'X' defined as a struct here but previously declared as a class; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]
struct X { int i; }; // now X is a complete type
^
p72.cpp:26:1: note: did you mean struct here?
class X; // X is an incomplete type
^~~~~
struct
p72.cpp:44:5: error: arithmetic on a pointer to an incomplete type 'UNKA' (aka 'int[]')
arrp++; // error: UNKA can’t be completed
~~~~^
1 warning and 15 errors generated.

$ g++ p72.cpp -std=03 -o p72g -I. -Wall
p72.cpp:13:1: warning: identifier 'constexpr' is a keyword in C++11 [-Wc++11-compat]
   13 | constexpr std::size_t N = sizeof(T);
      | ^~~~~~~~~
p72.cpp:13:1: error: 'constexpr' does not name a type
p72.cpp:13:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p72.cpp:14:10: error: 'N' was not declared in this scope
   14 | char buf[N];
      |          ^
p72.cpp:15:1: error: 'T' does not name a type
   15 | T obj; // obj initialized to its original value
      | ^
p72.cpp:16:12: error: expected constructor, destructor, or type conversion before '(' token
   16 | std::memcpy(buf, &obj, N); // between these two calls to std::memcpy, obj might be modified
      |            ^
p72.cpp:17:12: error: expected constructor, destructor, or type conversion before '(' token
   17 | std::memcpy(&obj, buf, N); // at this point, each subobject of obj of scalar type holds its original value
      |            ^
p72.cpp:19:1: error: 'T' does not name a type
   19 | T* t1p;
      | ^
p72.cpp:20:1: error: 'T' does not name a type
   20 | T* t2p;
      | ^
p72.cpp:22:12: error: expected constructor, destructor, or type conversion before '(' token
   22 | std::memcpy(t1p, t2p, sizeof(T));
      |            ^
p72.cpp: In function 'void foo()':
p72.cpp:33:1: error: cannot increment a pointer to incomplete type 'X'
   33 | xp++; // error: X is incomplete
      | ^~
p72.cpp:34:1: error: cannot increment a pointer to incomplete type 'UNKA' {aka 'int []'}
   34 | arrp++; // error: incomplete type
      | ^~~~
p72.cpp: In function 'void bar()':
p72.cpp:44:1: error: cannot increment a pointer to incomplete type 'UNKA' {aka 'int []'}
   44 | arrp++; // error: UNKA can’t be completed
      | ^~~~
rm: cannot remove 'p72g': No such file or directory

$ g++ p72.cpp -std=2b -o p72g -I. -Wall
p72.cpp:13:34: error: 'T' was not declared in this scope
   13 | constexpr std::size_t N = sizeof(T);
      |                                  ^
p72.cpp:14:10: error: size of array 'buf' is not an integral constant-expression
   14 | char buf[N];
      |          ^
p72.cpp:15:1: error: 'T' does not name a type
   15 | T obj; // obj initialized to its original value
      | ^
p72.cpp:16:12: error: expected constructor, destructor, or type conversion before '(' token
   16 | std::memcpy(buf, &obj, N); // between these two calls to std::memcpy, obj might be modified
      |            ^
p72.cpp:17:12: error: expected constructor, destructor, or type conversion before '(' token
   17 | std::memcpy(&obj, buf, N); // at this point, each subobject of obj of scalar type holds its original value
      |            ^
p72.cpp:19:1: error: 'T' does not name a type
   19 | T* t1p;
      | ^
p72.cpp:20:1: error: 'T' does not name a type
   20 | T* t2p;
      | ^
p72.cpp:22:12: error: expected constructor, destructor, or type conversion before '(' token
   22 | std::memcpy(t1p, t2p, sizeof(T));
      |            ^
p72.cpp: In function 'void foo()':
p72.cpp:33:1: error: cannot increment a pointer to incomplete type 'X'
   33 | xp++; // error: X is incomplete
      | ^~
p72.cpp:34:1: error: cannot increment a pointer to incomplete type 'UNKA' {aka 'int []'}
   34 | arrp++; // error: incomplete type
      | ^~~~
p72.cpp: In function 'void bar()':
p72.cpp:44:1: error: cannot increment a pointer to incomplete type 'UNKA' {aka 'int []'}
   44 | arrp++; // error: UNKA can’t be completed
      | ^~~~  
```

## 検討事項(agenda)
意味のある出力。

<|--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)

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

https://qiita.com/kaizen_nagoya/items/acb9f1a9b5aad6df06bd

dockerにclang

https://qiita.com/kaizen_nagoya/items/8829ffeee397eda50e80

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

https://qiita.com/drafts/059874ea39c4de64c0f7

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

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

https://qiita.com/kaizen_nagoya/items/483246d40f98abff7ded

C++ sample list

https://qiita.com/kaizen_nagoya/items/54ad75b73825b7c04f86


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 <misra_c.h>
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. 0.01 初稿 　20220620