Effective C++ Third version, sample code compile list(3)Use const whenever possible.

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)

```c++:ec3.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>

using namespace std;

const char * msg ="Item 3: Use const whenever possible.";

/// From here "Effective C++

//const keyword is remarkably versatile.

//For pointers, you can specify whether the pointer itself is const, the data it points to is const, both, or neither:

char greeting[] = "Hello";
char* p = greeting;  // non-const pointer, non-const data
const char* p = greeting;  // non-const pointer, const data
char* const p = greeting;  // const pointer, non-const data
const char* const p = greeting;  // const pointer, const data
//Declaring an iterator const is like declaring a pointer const (T * const pointer), the iterator isn't allow to point to something different, but the thing it points to may be modified.

const std::vector<int>::iterator iter = vec.begin();
*iter = 10; // ok
++iter;  // error

std::vector<int>::const_iterator cIter = vec.begin();
*cIter = 10;  // error
++iter;  // fine
//Return value can be const. Help to reduce the incidence of client errors without giving up safety or efficiency.

class Rational { ... };
const Rational operator*(const Rational& lhs, const Rational& rhs);

Rational a, b, c;
(a*b) = c; // error
if (a*b = c) ...    // good to prevent this error, can be identified during compilation
//We can easily prevent the above mistakes. What we want is if (a*b == c)

//We can also have const member functions

class TextBlock {
public:
    const char& operator[](std::size_t position) const 
    { return text[position]; }
    char& operator[](std::size_t position)
    { return text[position]; }
private:
    std::string text;
};

TextBlock tb("Hello");
std::cout << tb[0];  // call non-const operator[]
const TextBlock ctb("World");
std::cout << ctb[0];  // call the const one

tb[0] = 'x';  // ok
ctb[0] = 'x';  // error
Will be met when we pass parameter using reference-to-const.

//Bitwise const and logical const

//Compilers enforce bitwise const, but we need logical const!

class CTextBlock {
public:
    char& operator[](std::size_t position) const  // inappropriate but bitwise const 
    { return pText[position]; }
};
//Really want to modify value in const member function? mutable!

//Avoid Duplicatoin in const and Non-const Member Functions

class TextBlock {
public:
    const char& operator[](std::size_t position) const 
    { return text[position]; }
    char& operator[](std::size_t position)
    {
        return const_cast<char&>(static_cast<const TextBlock&>(*this)[position]);
    }
private:
    std::string text;
};
//But I don't this is a good idea...

//Things to Remember

//Declaring something const helps compilers detect usage errors. const can be applied to objects at any cope, to function parameters and return types, and to member functions as a whole.

//Copmilers enforce bitwise constness, but you should program using conceptual constness.

//When const and non-const member functions have essentially identical implementations, code duplicatoin can be avoided by having the non-const version call the const version.


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

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

```shell-session:cpa.sh
$$ ../c1.sh ec3
$ clang++ ec3.cpp  -std=c++2a -Wall
ec3.cpp:18:13: error: redefinition of 'p' with a different type: 'const char *' vs 'char *'
const char* p = greeting;  // non-const pointer, const data
            ^
ec3.cpp:17:7: note: previous definition is here
char* p = greeting;  // non-const pointer, non-const data
      ^
ec3.cpp:19:13: error: redefinition of 'p' with a different type: 'char *const' vs 'char *'
char* const p = greeting;  // const pointer, non-const data
            ^
ec3.cpp:17:7: note: previous definition is here
char* p = greeting;  // non-const pointer, non-const data
      ^
ec3.cpp:20:19: error: redefinition of 'p' with a different type: 'const char *const' vs 'char *'
const char* const p = greeting;  // const pointer, const data
                  ^
ec3.cpp:17:7: note: previous definition is here
char* p = greeting;  // non-const pointer, non-const data
      ^
ec3.cpp:23:12: error: implicit instantiation of undefined template 'std::__1::vector<int, std::__1::allocator<int> >'
const std::vector<int>::iterator iter = vec.begin();
           ^
/usr/local/Cellar/llvm/6.0.0/include/c++/v1/iosfwd:200:28: note: template is declared here
class _LIBCPP_TEMPLATE_VIS vector;
                           ^
ec3.cpp:23:25: error: use of class template 'iterator' requires template arguments
const std::vector<int>::iterator iter = vec.begin();
                        ^
/usr/local/Cellar/llvm/6.0.0/include/c++/v1/iterator:521:29: note: template is declared here
struct _LIBCPP_TEMPLATE_VIS iterator
                            ^
ec3.cpp:23:41: error: use of undeclared identifier 'vec'
const std::vector<int>::iterator iter = vec.begin();
                                        ^
ec3.cpp:24:2: error: C++ requires a type specifier for all declarations
*iter = 10; // ok
 ^
ec3.cpp:25:1: error: expected unqualified-id
++iter;  // error
^
ec3.cpp:27:6: error: implicit instantiation of undefined template 'std::__1::vector<int, std::__1::allocator<int> >'
std::vector<int>::const_iterator cIter = vec.begin();
     ^
/usr/local/Cellar/llvm/6.0.0/include/c++/v1/iosfwd:200:28: note: template is declared here
class _LIBCPP_TEMPLATE_VIS vector;
                           ^
ec3.cpp:27:42: error: use of undeclared identifier 'vec'
std::vector<int>::const_iterator cIter = vec.begin();
                                         ^
ec3.cpp:28:2: error: C++ requires a type specifier for all declarations
*cIter = 10;  // error
 ^
ec3.cpp:29:1: error: expected unqualified-id
++iter;  // fine
^
ec3.cpp:32:22: error: expected member name or ';' after declaration specifiers
class Rational { ... };
                     ^
ec3.cpp:36:3: error: expected ')'
(a*b) = c; // error
  ^
ec3.cpp:36:1: note: to match this '('
(a*b) = c; // error
^
ec3.cpp:36:2: error: C++ requires a type specifier for all declarations
(a*b) = c; // error
 ^
ec3.cpp:37:1: error: expected unqualified-id
if (a*b = c) ...    // good to prevent this error, can be identified during compilation
^
ec3.cpp:52:1: error: unknown type name 'TextBlock'
TextBlock tb("Hello");
^
ec3.cpp:53:6: error: no type named 'cout' in namespace 'std'
std::cout << tb[0];  // call non-const operator[]
~~~~~^
ec3.cpp:53:11: error: expected unqualified-id
std::cout << tb[0];  // call non-const operator[]
          ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.
```
#算譜(source code)

```c++:ec3.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 3: Use const whenever possible.";

/// From here "Effective C++

//const keyword is remarkably versatile.

//For pointers, you can specify whether the pointer itself is const, the data it points to is const, both, or neither:

char greeting[] = "Hello";
char* p1 = greeting;  // non-const pointer, non-const data /// change name p to p1
const char* p2 = greeting;  // non-const pointer, const data/// change name p to p2
char* const p3 = greeting;  // const pointer, non-const data /// change name p to p3
const char* const p4 = greeting;  // const pointer, const data ///change name p to p4
//Declaring an iterator const is like declaring a pointer const (T * const pointer), the iterator isn't allow to point to something different, but the thing it points to may be modified.///change name p to p5

void f() { /// add {}
  vector<int> vec = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};/// add definition vec
  const std::vector<int>::iterator iter = vec.begin();
  *iter = 10; // ok
///++iter;  // error ///ec3.cpp:28:1: error: cannot increment value of type 'const std::vector<int>::iterator' (aka 'const __wrap_iter<int *>')


  std::vector<int>::const_iterator cIter = vec.begin();
/// *cIter = 10;  // error ///ec3.cpp:31:8: error: cannot assign to return value because function 'operator*' returns a const value
  ++iter;  // fine
//Return value can be const. Help to reduce the incidence of client errors without giving up safety or efficiency.
}/// add {}

class Rational {
///...
};
const Rational operator*(const Rational& lhs, const Rational& rhs);

void h() {
  Rational a, b, c;
/// (a*b) = c; // error ec3.cpp:43:7: error: no viable overloaded '='
  if (a*b = c)
  }
/// ...    // good to prevent this error, can be identified during compilation
//We can easily prevent the above mistakes. What we want is if (a*b == c)

//We can also have const member functions

class TextBlock {
public:
  const char& operator[](std::size_t position) const
  {
    return text[position];
  }
  char& operator[](std::size_t position)
  {
    return text[position];
  }
private:
  std::string text;
};

void g() {
  TextBlock tb("Hello");
  std::cout << tb[0];  // call non-const operator[]
  const TextBlock ctb("World");
  std::cout << ctb[0];  // call the const one

  tb[0] = 'x';  // ok
  ctb[0] = 'x';  // error
//Will be met when we pass parameter using reference-to-const.
}
//Bitwise const and logical const

//Compilers enforce bitwise const, but we need logical const!

class CTextBlock {
public:
  char& operator[](std::size_t position) const  // inappropriate but bitwise const
  {
    return pText[position];
  }
};
//Really want to modify value in const member function? mutable!

//Avoid Duplicatoin in const and Non-const Member Functions

class TextBlock {
public:
  const char& operator[](std::size_t position) const
  {
    return text[position];
  }
  char& operator[](std::size_t position)
  {
    return const_cast<char&>(static_cast<const TextBlock&>(*this)[position]);
  }
private:
  std::string text;
};
//But I don't this is a good idea...

//Things to Remember

//Declaring something const helps compilers detect usage errors. const can be applied to objects at any cope, to function parameters and return types, and to member functions as a whole.

//Copmilers enforce bitwise constness, but you should program using conceptual constness.

//When const and non-const member functions have essentially identical implementations, code duplicatoin can be avoided by having the non-const version call the const version.


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

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

```
#編纂・実行結果(compile and go)

```shell-session:cpa.sh
$ ../c1.sh ec3
$ clang++ ec3.cpp  -std=c++2a -Wall
ec3.cpp:33:1: error: cannot increment value of type 'const std::vector<int>::iterator' (aka 'const __wrap_iter<int *>')
++iter;  // fine
^ ~~~~
ec3.cpp:45:9: error: no viable overloaded '='
if (a*b = c)
    ~~~ ^ ~
ec3.cpp:37:7: note: candidate function (the implicit copy assignment operator) not viable: 'this' argument has type
      'const Rational', but method is not marked const
class Rational { 
      ^
ec3.cpp:37:7: note: candidate function (the implicit move assignment operator) not viable: 'this' argument has type
      'const Rational', but method is not marked const
ec3.cpp:46:1: error: expected statement
}
^
ec3.cpp:63:11: error: no matching constructor for initialization of 'TextBlock'
TextBlock tb("Hello");
          ^  ~~~~~~~
ec3.cpp:52:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'const char [6]'
      to 'const TextBlock' for 1st argument
class TextBlock {
      ^
ec3.cpp:52:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'const char [6]'
      to 'TextBlock' for 1st argument
class TextBlock {
      ^
ec3.cpp:52:7: note: candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was
      provided
ec3.cpp:65:17: error: no matching constructor for initialization of 'const TextBlock'
const TextBlock ctb("World");
                ^   ~~~~~~~
ec3.cpp:52:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'const char [6]'
      to 'const TextBlock' for 1st argument
class TextBlock {
      ^
ec3.cpp:52:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'const char [6]'
      to 'TextBlock' for 1st argument
class TextBlock {
      ^
ec3.cpp:52:7: note: candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was
      provided
ec3.cpp:79:14: error: use of undeclared identifier 'pText'
    { return pText[position]; }
             ^
ec3.cpp:85:7: error: redefinition of 'TextBlock'
class TextBlock {
      ^
ec3.cpp:52:7: note: previous definition is here
class TextBlock {
      ^
7 errors generated.
```

# 参考資料(reference)
Effective C++（項1〜5）解説
https://qiita.com/MoriokaReimen/items/58f183d421bb932cbbda
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

# 文書履歴(document history)
ver. 0.10 初稿 20180619
ver. 0.11 参考文献追記 20180623
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