Autosar Guidelines C++14, example code compile list(57)Rule A7-1-2The constexpr specifier 

Guidelines for the use of the C++14 language in critical and
safety-related systems Sample code compile list(57)
https://www.autosar.org/fileadmin/user_upload/standards/adaptive/17-03/AUTOSAR_RS_CPP14Guidelines.pdf
Autosar Guidelines C++14, example code compile list(
#目的(purpose)
AutosarのC++ GuidelineをOS, 制御のプログラムで利用するにあたって、(1)hosted, freestandingのどちらを基本にすべきか。(2)C++2014,C++2017, C++202aのどれを用いると良いか, (3)どの処理系を併用すると良いかを検討するため、-std=c++14, -std=c++17, -std=c++2aの3種類で、複数のコンパイラでコンパイルすることにより、誤(error)、警告(warning)、関数・変数連携(link)、出力(output)、にどのような影響があるかを確認する。
#成果(outcome)
複数の処理系の特徴が明確になるとともに、各標準段階, hosted, freestandingの特徴と課題を明確にする。


#A7-1-2.cpp
##算譜(source code)

```c++:A7-1-2.cpp
//Guidelines for the use of the C++14 language in critical and safety-related systems
const char* msg="Rule A7-1-2 (required, implementation, automated)The constexpr specifier shall be used for values that can be determined at compile time.(57)A7-1-2.cpp";
//https://www.autosar.org/fileadmin/user_upload/standards/adaptive/17-03/AUTOSAR_RS_CPP14Guidelines.pdf
// There is no description about Autosar declear hosted or freestanding.
// If the Autosar intended both depending on the cases, autosar.h can choose one.
// Compile with -DHOSTED work as  hosted environment, -DFREESTANDING work as freestanding.

#include "autosar.h"/// @line add header file https://qiita.com/kaizen_nagoya/items/4bde8f21ab059b96cf2a

using namespace std;/// @line add using

///start AUTOSAR: From here to the "///end AUTOSAR" is from AUTOSAR without code having /// comment in line.
//% $Id: A7-1-2.cpp 271715 2017-03-23 10:13:51Z piotr.tanski $
#include <cstdint>
std::int32_t pow1(std::int32_t number)
{
  return (number * number);
}
constexpr std::int32_t pow2(
  std::int32_t number) // Possible compile-time computing
// because of constexpr specifier
{
  return (number * number);
}
void fn()
{
  constexpr std::int16_t i1 = 20; // Compliant, evaluated at compile-time
  const std::int16_t i2 = 20; // Non-compliant, possible run-time evaluation
  std::int32_t twoSquare =
    pow1(2); // Non-compliant, possible run-time evaluation
  const std::int32_t threeSquare =
    pow1(3); // Non-compliant, possible run-time evaluation
// static_assert(threeSquare == 9, "pow1(3) did not succeed."); // Value
// can not be static_assert-ed
  constexpr std::int32_t fiveSquare =
    pow2(5); // Compliant, evaluated at compile time
  static_assert(fiveSquare == 25,
                "pow2(5) did not succeed."); // Compliant, constexpr
// evaluated at compile time
// constexpr std::int32_t int32Max =
// std::numeric_limits<std::int32_t>::max() + 1; //
// Compliant - compilation error due to
// compile-time evaluation (integer overflow)
  cout <<"i1="<<i1<<" i2="<<i2<<" twoSquare="<<twoSquare<<" threeSquare="<<threeSquare<<endl;/// @ line for output
}
class A
{
public:
  static constexpr double pi = 3.14159265; // Compliant - value of PI can be
// determined in compile time

// constexpr double e = 2.71828182; // Non-compliant - constexprs need
// to be static members, compilation error

  constexpr A() = default; // Compliant
};
///end AUTOSAR
int start() { /// @{} for start
  fn();
  cout<< msg << endl;
  ShutdownOS()  EXIT_SUCCESS;
/// Autosar OS 3.1.1, 2009: 7.1.2.2 Undefined Behaviour in OSEK OS
/// OS425 If ShutdownOS is called and ShutdownHook() returns then the operating system shall disable all interrupts and enter an endless loop.
}
```

##編纂・実行結果(compile and go)

```shell-session:cpa.sh
$ ../cpa.sh a7-1-2
$ clang++ a7-1-2.cpp -I./ -std=c++14 -Wall
i1=20 i2=20 twoSquare=4 threeSquare=9
Rule A7-1-2 (required, implementation, automated)The constexpr specifier shall be used for values that can be determined at compile time.(57)A7-1-2.cpp
$ clang++ a7-1-2.cpp  -I./ -std=c++17 -Wall
i1=20 i2=20 twoSquare=4 threeSquare=9
Rule A7-1-2 (required, implementation, automated)The constexpr specifier shall be used for values that can be determined at compile time.(57)A7-1-2.cpp
$ clang++ a7-1-2.cpp  -I./ -std=c++2a -Wall
i1=20 i2=20 twoSquare=4 threeSquare=9
Rule A7-1-2 (required, implementation, automated)The constexpr specifier shall be used for values that can be determined at compile time.(57)A7-1-2.cpp

$ g++-8 a7-1-2.cpp  -I./ -std=c++14  -Wall
i1=20 i2=20 twoSquare=4 threeSquare=9
Rule A7-1-2 (required, implementation, automated)The constexpr specifier shall be used for values that can be determined at compile time.(57)A7-1-2.cpp
$ g++-8 a7-1-2.cpp  -I./ -std=c++17  -Wall
i1=20 i2=20 twoSquare=4 threeSquare=9
Rule A7-1-2 (required, implementation, automated)The constexpr specifier shall be used for values that can be determined at compile time.(57)A7-1-2.cpp
$ g++-8 a7-1-2.cpp  -I./ -std=c++2a  -Wall
i1=20 i2=20 twoSquare=4 threeSquare=9
Rule A7-1-2 (required, implementation, automated)The constexpr specifier shall be used for values that can be determined at compile time.(57)A7-1-2.cpp
```

#検討事項(agenda)
###1. 自律(freestanding)環境. 接待(hosted)環境

C++N4606 1.4 Implementation compliance p.4

###2. 対応OSの水準、対応通信規約、応用機能による分類
freestanding用の関数、ライブラリ等
###3. C++2014, C++2017, C++202aの比較項目
本件なし
###4. clang++, g++の比較検討項目
本件なし

###5 役立つまたは意味のある出力

#参考文献(reference)
###C++N4741 2018
Working Draft, Standard for Programming Language C++
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4741.pdf

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

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

C++N4606, 2016符号断片編纂一覧(example code compile list)
 Working Draft 2016, ISO/IEC 14882(1)
https://qiita.com/kaizen_nagoya/items/df5d62c35bd6ed1c3d43/

#文書履歴
ver 0.10 初稿 20180610