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@kaizen_nagoya(Dr. Kiyoshi Ogawa)

Effective C++ Third version, sample code compile list(8) Prevent exceptions from leaving your destructors

Last updated at Posted at 2018-06-22

Effective C++: 55 Specific Ways to Improve Your Programs and DesignsThird version by Scott Mayers
https://www.informit.com/store/effective-c-plus-plus-55-specific-ways-to-improve-your-9780321334879
https://www.amazon.com/dp/0321334876

Sample code from
Yuzhen11/Effective_CPP
https://github.com/Yuzhen11/Effective_CPP

目的(purpose)

"Effective C++コンパイル記録は、コンパイラおよび対応標準により、コンパイルエラーMessageの違いを記録し、どのエラーが出たら、どの対応標準にすればエラーが少なくなるかを考察するための資料の第一歩です。
上記参照サイトにはコンパイルする仕組みを提供していない。簡易な台本(script)を作成する。

成果(outcome)

計画(plan)

(1)コンパイラの種類、対応標準の違いによってエラーの数が違う。
(2)同じエラー、警告であってもMessageの表現が違う。
(3) エラー、警告のMessageをネットで検索する際に役立てる。
(4)コード断片の役に立つまたは意味のあるコンパイル・リンク・実行エラーの取り方を検討する
(5)コード断片の役に立つまたは意味のある出力を検討する

結果(result)現在整理中

  1. コンパイラによるエラー、渓谷の違い
  2. コンパイラによる必要なヘッダファイルの違い
  3. 参考にした資料

算譜(source code)

ec8.cpp
///Effective C++:  55 Specific Ways to Improve Your Programs and DesignsThird version by Scott Mayers
///https://www.informit.com/store/effective-c-plus-plus-55-specific-ways-to-improve-your-9780321334879
#include <iostream>
#include <cstdlib>
#include <vector>
using namespace std;

const char * msg ="Item 8: Prevent exceptions from leaving your destructors";

/// From here "Effective C++

//C++ doesn't prohibit destructors from emitting exceptions, but it certainly discourages the practice.

class Widget {
public:
  ///  ...
  ~Widget() {
///...
  }
};
void doSomething() {
  std::vector<Widget> v;
}
//Possible that two simultaneously active exceptions throw.

//C++ does not liek destructors that emit exceptions!

class DBConnection {
public:
///   ...
  static DBConnection create();
  void close() {};  // close connection; throw an exception if closing fails
};
namespace A {
  class DBConn {
  public:
    ~DBConn() {
      db.close();
    }
  private:
    DBConnection db;
  };
}
//What if the destructor throws exceptions?

//here are two primary ways to avoid the trouble.

namespace B {
  class DBConn {
  public:
    ~DBConn();
  private:
    DBConnection db;
  };

//Terminate the program if close throws
  DBConn::~DBConn() {
    try {
      db.close();
    }
    catch(logic_error& e/*...*/) {
      //   make log entry that the call to close failed
      std::abort();
    }
  }
}

namespace C {
  class DBConn {
  public:
    ~DBConn();
  private:
    DBConnection db;
  };


//Swallow the exception
  DBConn::~DBConn() {
    try {
      db.close();
    }
    catch(logic_error& e/*...*/) {
      //      make log entry that the call to close failed
    }
  }
}
//Neither of these approaches is especially appealing. The problem with both is that the program has no way to react to the condition that led to close throwing an exception in the first place.

//A better strategy is to design DBConn's interface so that its clients have an opportunity to react to problems that may arise.

//Could offer a close function itself. thus giveing clients a chance to handle exceptions arising from that operation.

//It could also keep track of wheter its DBConnection had been closed, closing it itself in the destructor if not. That would prevent leaking.

//If the call to close were to fail in DBConnection destructor, we'd be back to terminating or swallowing:
namespace D {
  class DBConn {
  public:
    void close() {
      db.close();
      closed = true;
    }
    ~DBConn() {
      if (!closed) {
        try {
          db.close();
        }
        catch(logic_error& e/*...*/) {
//                make log entry that call to close failed
          //...
        }
      }
    }
  private:
    DBConnection db;
    bool closed;
  };
}
//Telling cleints to call close themselves doesn't impose a burden on them; it gives them an opportunity to deal with errors they would otherwise have no chance to react to .

//Things to Remember

//Destructors should never emit exceptions. If functions called in a destructor may throw, the destructor should catch any exceptions, then swallow them or terminate the program.

//If class clients need to be able to react to exceptions thrown during an operation, the class should provide a regular (i.e., non-destructor) function that performs the operation.

/// to here from "Effective C++"

int main() {
  cout << msg << endl;
  return EXIT_SUCCESS;
}

編纂・実行結果(compile and go)

cpa.sh
$ ../cpa.sh ec8
$ clang++ ec8.cpp -I./ -std=c++14 -Wall
Item 8: Prevent exceptions from leaving your destructors
$ clang++ ec8.cpp  -I./ -std=c++17 -Wall
Item 8: Prevent exceptions from leaving your destructors
$ clang++ ec8.cpp  -I./ -std=c++2a -Wall
Item 8: Prevent exceptions from leaving your destructors

$ g++-8 ec8.cpp  -I./ -std=c++14  -Wall
Item 8: Prevent exceptions from leaving your destructors
$ g++-8 ec8.cpp  -I./ -std=c++17  -Wall
Item 8: Prevent exceptions from leaving your destructors
$ g++-8 ec8.cpp  -I./ -std=c++2a  -Wall
Item 8: Prevent exceptions from leaving your destructors

課題(agenda)

1)役に立つまたは意味のある出力
2)役に立つまたは意味のある関数、クラスの呼び出し方

参考資料(reference)

Effective C++(項6〜10)解説
https://qiita.com/MoriokaReimen/items/bf301530e8f5d6d2fe90
Effective C++ Third version, sample code compile list(まとめ）
https://qiita.com/kaizen_nagoya/items/2092638c221e1033040c

docker gnu(gcc/g++) and llvm(clang/clang++)
https://qiita.com/drafts/059874ea39c4de64c0f7
[C][C++]の国際規格案の例題をコンパイルするときの課題７つ。
https://qiita.com/kaizen_nagoya/items/5f4b155030259497c4de
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
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
C++参考資料一覧
https://researchmap.jp/joub9b3my-1797580/#_1797580

文書履歴(document history)

ver. 0.10 初稿 20180622
ver. 0.11 参考文献追記 20180623

最後までおよみいただきありがとうございました。

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Thank you very much for reading to the last sentence.

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