Effective C++ Third version, sample code compile list(11) Handle assignment to self in operator=

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


#算譜(source code)

```c++:ec11.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 11: Handle assignment to self in operator=";

/// From here "Effective C++
namespace A {
  class Widget {
///...
  };

  void f() {
    Widget w;
///...
    w = w;
    int i=8;
    int j=9;
    int k=10;
    int a[k];
    int * px=&i;
    int * py=&j;
    a[i] = a[j];  // potential assignment to self
    *px = *py;  // potential assignment to self
//If you try to manage resources yourself, you can fall into the trap of accidentally releasing a resource before you're done using it.
  }
}

namespace B {
  class Bitmap {
/// ...
  };
  class Widget {
///    ...
  private:
    Bitmap* pb;
  };
//Here's an unsafe implementation:

  Widget& Widget::operator=(const Widget& rhs) {
    delete pb;
    pb = new Bitmap(*rhs.ph);
    return *this;
  }
//Traditional way to prevent this error:

  Widget& Widget::operator=(const Widget& rhs) {
    if (this == &rhs) return *this;
    delete pb;
    pb = new Bitmap(*rhs.ph);
    return *this;
  }
//This version still has exception trouble. What if new yield an exception, the Widget will end up holding a pointer to a deleted Bitmap.

//Happily, making operator= exeption-safe typically redners it self-assignment-safe too.

//A careful ordering of statements can yield exception-safe.

  Widget& Widget::operator=(const Widget& rhs) {
    Bitmap* pOrig = pb;  // remember the original pb
    pb = new Bitmap(*rhs.pb);
    delete pOrig;  // delete the original pb
    return *this;
  }
}
//Now, if new Bitmap throws an excpetion, pb remains unchanged. This code handle self-assignement too but with less efficient(delete and new).

//copy and swap
namespace D {
  class Widget {
    void swap(Widget& rhs);
  };
  Widget& Widget::operator=(const Widget& rhs) {
    Widget temp(rhs);
    swap(temp);
    return *this;
  }
// A variation, sacrifices clarity at the altar of cleverness.
  Widget& Widget::operator=(Widget rhs) {
    swap(rhs);
    return *this;
  }
}
//Things to Remember

//Make sure operator= is well-behaved when an object is assigned to itself. Technqiues include comparing addresses of source and target objects, careful statement ordering, and cpoy-and-swap.
//Make suret that any function operating on more than one object behaves correctly if two or more of the objects are the same.

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

int main() {
  A::f();
  cout << msg << endl;
  return EXIT_SUCCESS;
}```
#編纂・実行結果(compile and go)

```shell-session:c1.sh
$ ../c1.sh ec11
$ clang++ ec11.cpp  -std=c++2a -Wall
ec11.cpp:43:17: error: 'operator=' is missing exception specification 'noexcept'
Widget& Widget::operator=(const Widget& rhs) {
                ^
                                             noexcept
ec11.cpp:36:7: note: previous declaration is here
class Widget {
      ^
ec11.cpp:45:26: error: no member named 'ph' in 'B::Widget'
    pb = new Bitmap(*rhs.ph);
                     ~~~ ^
ec11.cpp:50:17: error: 'operator=' is missing exception specification 'noexcept'
Widget& Widget::operator=(const Widget& rhs) {
                ^
                                             noexcept
ec11.cpp:36:7: note: previous declaration is here
class Widget {
      ^
ec11.cpp:53:26: error: no member named 'ph' in 'B::Widget'
    pb = new Bitmap(*rhs.ph);
                     ~~~ ^
ec11.cpp:62:17: error: 'operator=' is missing exception specification 'noexcept'
Widget& Widget::operator=(const Widget& rhs) {
                ^
                                             noexcept
ec11.cpp:36:7: note: previous declaration is here
class Widget {
      ^
ec11.cpp:76:17: error: 'operator=' is missing exception specification 'noexcept'
Widget& Widget::operator=(const Widget& rhs) {
                ^
                                             noexcept
ec11.cpp:73:7: note: previous declaration is here
class Widget {
      ^
```
#課題(agenda)
1)役に立つまたは意味のあるコンパイルエラーの取り方

2)役に立つまたは意味のある出力

3)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)

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
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https://qiita.com/kaizen_nagoya/items/74220c0577a512c2d7da
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https://qiita.com/kaizen_nagoya/items/9a82b958cc3aeef0403f
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https://qiita.com/kaizen_nagoya/items/a13ea3823441c430edff
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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
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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