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@kaizen_nagoya(Dr. Kiyoshi Ogawa)

9.4.4 References [dcl.init.ref] C++N4910:2022 (105) p210.cpp

Posted at

はじめに(Introduction)

N4910 Working Draft, Standard for Programming Language C++

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。

<この項は書きかけです。順次追記します。>

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

C++N4741, 2018 Standard Working Draft on ISO/IEC 14882 sample code compile list

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++

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.

9.4.4 References [dcl.init.ref] C++N4910:2022 (105) p210.cpp

算譜(source code)

p210.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 * n4910 = "9.4.4 References [dcl.init.ref] C++N4910:2022 (105) p210.cpp";
// Debian clang version 14.0.5-++20220610033153+c12386ae247c-
// g++ (GCC) 12.1.0 Copyright (C) 2022 Free Software Foundation, Inc.
// Edited by Dr. Ogawa Kiyoshi. Compile procedure and results record.
// C++N4910:2022 Standard Working Draft on ISO/IEC 14882(0) sample code compile list
// https://qiita.com/kaizen_nagoya/items/fc957ddddd402004bb91

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <coroutine>

using namespace std;

// Example 1
int g(int) noexcept;
void f() {
    int i;
    int& r = i; // r refers to i
    r = 1; // the value of i becomes 1
    int* p = &r; // p points to i
    int& rr = r; // rr refers to what r refers to, that is, to i
    int (&rg)(int) = g; // rg refers to the function g
    rg(i); // calls function g
    int a[3];
    int (&ra)[3] = a; // ra refers to the array a
    ra[1] = i; // modifies a[1]
}
// [Example 2 :
int& r1; // error: initializer missing
extern int& r2; // OK
// [Example 3 :
double d = 2.0;
double& rd = d; // rd refers to d
const double& rcd = d; // rcd refers to d
struct A { };
struct B : A {
    operator int&();
} b;
A& ra = b; // ra refers to A subobject in b
const A& rca = b; // rca refers to A subobject in b
int& ir = B(); // ir refers to the result of B::operator int&
// [Example 4 :
double& rd2 = 2.0; // error: not an lvalue and reference not const
int i4 = 2;
double& rd3 = i4; // error: type mismatch and reference not const
// [Example 5 :
struct A5 { };
struct B5 : A5 { } b5;
extern B5 f5();
const A5& rca2 = f5(); // binds to the A subobject of the B rvalue.
A&& rra = f5(); // same as above
struct X5 {
    operator B5();
    operator int&();
} x5;
const A5& r5 = x5; // binds to the A subobject of the result of the conversion
int i2 = 42;
int&& rri = static_cast<int&&>(i2); // binds directly to i2
B5&& rrb = x5; // binds directly to the result of operator B
// [Example 6 :
struct Banana { };
struct Enigma {
    operator const Banana();
};
struct Alaska {
    operator Banana&();
};
void enigmatic() {
    typedef const Banana ConstBanana;
    Banana &&banana1 = ConstBanana(); // error
    Banana &&banana2 = Enigma(); // error
    Banana &&banana3 = Alaska(); // error
}
const double& rcd26 = 2; // rcd2 refers to temporary with value 2.0
double&& rrd = 2; // rrd refers to temporary with value 2.0
const volatile int cvi = 1;
const int& r26 = cvi; // error: cv-qualifier dropped
struct A6 {
    operator volatile int&();
} a6;
const int& r3 = a6; // error: cv-qualifier dropped
// from result of conversion function
double d2 = 1.0;
double&& rrd26 = d2; // error: initializer is lvalue of related type
struct X6 {
    operator int&();
};
int&& rri2 = X6(); // error: result of conversion function is lvalue of related type
int i36 = 2;
double&& rrd3 = i36; // rrd3 refers to temporary with value 2.0
int main() {
    cout  <<  n4910 << endl;
    return EXIT_SUCCESS;
}

Script

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)

bash
# ./clgc.sh  p210
rm: cannot remove 'p210l': No such file or directory
rm: cannot remove 'p210g': No such file or directory
$ clang++ p210.cpp -std=03 -o p210l -I. -Wall
p210.cpp:20:12: error: expected function body after function declarator
int g(int) noexcept;
           ^
p210.cpp:27:18: error: use of undeclared identifier 'g'
int (&rg)(int) = g; // rg refers to the function g
                 ^
p210.cpp:34:6: error: declaration of reference variable 'r1' requires an initializer
int& r1; // error: initializer missing
     ^~
p210.cpp:46:9: error: non-const lvalue reference to type 'double' cannot bind to a temporary of type 'double'
double& rd2 = 2.0; // error: not an lvalue and reference not const
        ^     ~~~
p210.cpp:48:9: error: non-const lvalue reference to type 'double' cannot bind to a value of unrelated type 'int'
double& rd3 = i4; // error: type mismatch and reference not const
        ^     ~~
p210.cpp:54:2: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
A&& rra = f5(); // same as above
 ^
p210.cpp:54:5: error: no viable conversion from 'B5' to 'A'
A&& rra = f5(); // same as above
    ^     ~~~~
p210.cpp:40:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'B5' to 'const A &' for 1st argument
struct A { };
       ^
p210.cpp:61:4: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
int&& rri = static_cast<int&&>(i2); // binds directly to i2
   ^
p210.cpp:61:28: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
int&& rri = static_cast<int&&>(i2); // binds directly to i2
                           ^
p210.cpp:62:3: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
B5&& rrb = x5; // binds directly to the result of operator B
  ^
p210.cpp:69:8: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
Banana &&banana1 = ConstBanana(); // error
       ^
p210.cpp:69:10: error: binding reference of type 'Banana' to value of type 'ConstBanana' (aka 'const Banana') drops 'const' qualifier
Banana &&banana1 = ConstBanana(); // error
         ^         ~~~~~~~~~~~~~
p210.cpp:70:8: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
Banana &&banana2 = Enigma(); // error
       ^
p210.cpp:70:10: error: no viable conversion from 'Enigma' to 'Banana'
Banana &&banana2 = Enigma(); // error
         ^         ~~~~~~~~
p210.cpp:64:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'Enigma' to 'const Banana &' for 1st argument
struct Banana { };
       ^
p210.cpp:65:17: note: candidate function
struct Enigma { operator const Banana(); };
                ^
p210.cpp:71:8: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
Banana &&banana3 = Alaska(); // error
       ^
p210.cpp:71:10: error: no viable conversion from 'Alaska' to 'Banana'
Banana &&banana3 = Alaska(); // error
         ^         ~~~~~~~~
p210.cpp:64:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'Alaska' to 'const Banana &' for 1st argument
struct Banana { };
       ^
p210.cpp:66:17: note: candidate function
struct Alaska { operator Banana&(); };
                ^
p210.cpp:74:7: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
double&& rrd = 2; // rrd refers to temporary with value 2.0
      ^
p210.cpp:76:12: error: binding reference of type 'const int' to value of type 'const volatile int' drops 'volatile' qualifier
const int& r26 = cvi; // error: cv-qualifier dropped
           ^     ~~~
p210.cpp:78:12: error: no viable conversion from 'struct A6' to 'const int'
const int& r3 = a6; // error: cv-qualifier dropped
           ^    ~~
p210.cpp:77:13: note: candidate function
struct A6 { operator volatile int&(); } a6;
            ^
p210.cpp:81:7: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
double&& rrd26 = d2; // error: initializer is lvalue of related type
      ^
p210.cpp:81:10: error: rvalue reference to type 'double' cannot bind to lvalue of type 'double'
double&& rrd26 = d2; // error: initializer is lvalue of related type
         ^       ~~
p210.cpp:83:4: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
int&& rri2 = X6(); // error: result of conversion function is lvalue of related type
   ^
p210.cpp:83:7: error: no viable conversion from 'X6' to 'int'
int&& rri2 = X6(); // error: result of conversion function is lvalue of related type
      ^      ~~~~
p210.cpp:82:13: note: candidate function
struct X6 { operator int&(); };
            ^
p210.cpp:85:7: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
double&& rrd3 = i36; // rrd3 refers to temporary with value 2.0
      ^
11 warnings and 13 errors generated.
rm: cannot remove 'p210l': No such file or directory
$ clang++ p210.cpp -std=2b -o p210l -I. -Wall
p210.cpp:26:6: warning: unused variable 'rr' [-Wunused-variable]
int& rr = r; // rr refers to what r refers to, that is, to i
     ^
p210.cpp:25:6: warning: unused variable 'p' [-Wunused-variable]
int* p = &r; // p points to i
     ^
p210.cpp:34:6: error: declaration of reference variable 'r1' requires an initializer
int& r1; // error: initializer missing
     ^~
p210.cpp:46:9: error: non-const lvalue reference to type 'double' cannot bind to a temporary of type 'double'
double& rd2 = 2.0; // error: not an lvalue and reference not const
        ^     ~~~
p210.cpp:48:9: error: non-const lvalue reference to type 'double' cannot bind to a value of unrelated type 'int'
double& rd3 = i4; // error: type mismatch and reference not const
        ^     ~~
p210.cpp:54:5: error: no viable conversion from 'B5' to 'A'
A&& rra = f5(); // same as above
    ^     ~~~~
p210.cpp:40:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'B5' to 'const A &' for 1st argument
struct A { };
       ^
p210.cpp:40:8: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'B5' to 'A &&' for 1st argument
struct A { };
       ^
p210.cpp:69:10: error: binding reference of type 'Banana' to value of type 'ConstBanana' (aka 'const Banana') drops 'const' qualifier
Banana &&banana1 = ConstBanana(); // error
         ^         ~~~~~~~~~~~~~
p210.cpp:70:10: error: no viable conversion from 'Enigma' to 'Banana'
Banana &&banana2 = Enigma(); // error
         ^         ~~~~~~~~
p210.cpp:64:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'Enigma' to 'const Banana &' for 1st argument
struct Banana { };
       ^
p210.cpp:64:8: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'Enigma' to 'Banana &&' for 1st argument
struct Banana { };
       ^
p210.cpp:65:17: note: candidate function
struct Enigma { operator const Banana(); };
                ^
p210.cpp:71:10: error: no viable conversion from 'Alaska' to 'Banana'
Banana &&banana3 = Alaska(); // error
         ^         ~~~~~~~~
p210.cpp:64:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'Alaska' to 'const Banana &' for 1st argument
struct Banana { };
       ^
p210.cpp:64:8: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'Alaska' to 'Banana &&' for 1st argument
struct Banana { };
       ^
p210.cpp:66:17: note: candidate function
struct Alaska { operator Banana&(); };
                ^
p210.cpp:76:12: error: binding reference of type 'const int' to value of type 'const volatile int' drops 'volatile' qualifier
const int& r26 = cvi; // error: cv-qualifier dropped
           ^     ~~~
p210.cpp:78:12: error: no viable conversion from 'struct A6' to 'const int'
const int& r3 = a6; // error: cv-qualifier dropped
           ^    ~~
p210.cpp:77:13: note: candidate function
struct A6 { operator volatile int&(); } a6;
            ^
p210.cpp:81:10: error: rvalue reference to type 'double' cannot bind to lvalue of type 'double'
double&& rrd26 = d2; // error: initializer is lvalue of related type
         ^       ~~
p210.cpp:83:7: error: no viable conversion from 'X6' to 'int'
int&& rri2 = X6(); // error: result of conversion function is lvalue of related type
      ^      ~~~~
p210.cpp:82:13: note: candidate function
struct X6 { operator int&(); };
            ^
2 warnings and 11 errors generated.

$ g++ p210.cpp -std=03 -o p210g -I. -Wall
p210.cpp:20:12: warning: identifier 'noexcept' is a keyword in C++11 [-Wc++11-compat]
   20 | int g(int) noexcept;
      |            ^~~~~~~~
p210.cpp:20:12: error: expected initializer before 'noexcept'
p210.cpp: In function 'void f()':
p210.cpp:27:18: error: 'g' was not declared in this scope; did you mean 'rg'?
   27 | int (&rg)(int) = g; // rg refers to the function g
      |                  ^
      |                  rg
p210.cpp:25:6: warning: unused variable 'p' [-Wunused-variable]
   25 | int* p = &r; // p points to i
      |      ^
p210.cpp:26:6: warning: unused variable 'rr' [-Wunused-variable]
   26 | int& rr = r; // rr refers to what r refers to, that is, to i
      |      ^~
p210.cpp: At global scope:
p210.cpp:34:6: error: 'r1' declared as reference but not initialized
   34 | int& r1; // error: initializer missing
      |      ^~
p210.cpp:46:15: error: cannot bind non-const lvalue reference of type 'double&' to an rvalue of type 'double'
   46 | double& rd2 = 2.0; // error: not an lvalue and reference not const
      |               ^~~
p210.cpp:48:15: error: cannot bind non-const lvalue reference of type 'double&' to a value of type 'int'
   48 | double& rd3 = i4; // error: type mismatch and reference not const
      |               ^~
p210.cpp:54:2: error: expected unqualified-id before '&&' token
   54 | A&& rra = f5(); // same as above
      |  ^~
p210.cpp:61:4: error: expected unqualified-id before '&&' token
   61 | int&& rri = static_cast<int&&>(i2); // binds directly to i2
      |    ^~
p210.cpp:62:3: error: expected unqualified-id before '&&' token
   62 | B5&& rrb = x5; // binds directly to the result of operator B
      |   ^~
p210.cpp: In function 'void enigmatic()':
p210.cpp:69:8: error: expected unqualified-id before '&&' token
   69 | Banana &&banana1 = ConstBanana(); // error
      |        ^~
p210.cpp:70:8: error: expected unqualified-id before '&&' token
   70 | Banana &&banana2 = Enigma(); // error
      |        ^~
p210.cpp:71:8: error: expected unqualified-id before '&&' token
   71 | Banana &&banana3 = Alaska(); // error
      |        ^~
p210.cpp: At global scope:
p210.cpp:74:7: error: expected unqualified-id before '&&' token
   74 | double&& rrd = 2; // rrd refers to temporary with value 2.0
      |       ^~
p210.cpp:76:18: error: binding reference of type 'const int&' to 'const volatile int' discards qualifiers
   76 | const int& r26 = cvi; // error: cv-qualifier dropped
      |                  ^~~
p210.cpp:78:17: error: invalid user-defined conversion from 'A6' to 'const int&' [-fpermissive]
   78 | const int& r3 = a6; // error: cv-qualifier dropped
      |                 ^~
p210.cpp:77:13: note: candidate is: 'A6::operator volatile int&()' (near match)
   77 | struct A6 { operator volatile int&(); } a6;
      |             ^~~~~~~~
p210.cpp:77:13: note:   no known conversion from 'volatile int&' to 'const int&'
p210.cpp:78:17: error: conversion from 'volatile int' to 'const int&' discards qualifiers [-fpermissive]
   78 | const int& r3 = a6; // error: cv-qualifier dropped
      |                 ^~
p210.cpp:81:7: error: expected unqualified-id before '&&' token
   81 | double&& rrd26 = d2; // error: initializer is lvalue of related type
      |       ^~
p210.cpp:83:4: error: expected unqualified-id before '&&' token
   83 | int&& rri2 = X6(); // error: result of conversion function is lvalue of related type
      |    ^~
p210.cpp:85:7: error: expected unqualified-id before '&&' token
   85 | double&& rrd3 = i36; // rrd3 refers to temporary with value 2.0
      |       ^~
rm: cannot remove 'p210g': No such file or directory

$ g++ p210.cpp -std=2b -o p210g -I. -Wall
p210.cpp: In function 'void f()':
p210.cpp:25:6: warning: unused variable 'p' [-Wunused-variable]
   25 | int* p = &r; // p points to i
      |      ^
p210.cpp:26:6: warning: unused variable 'rr' [-Wunused-variable]
   26 | int& rr = r; // rr refers to what r refers to, that is, to i
      |      ^~
p210.cpp: At global scope:
p210.cpp:34:6: error: 'r1' declared as reference but not initialized
   34 | int& r1; // error: initializer missing
      |      ^~
p210.cpp:46:15: error: cannot bind non-const lvalue reference of type 'double&' to an rvalue of type 'double'
   46 | double& rd2 = 2.0; // error: not an lvalue and reference not const
      |               ^~~
p210.cpp:48:15: error: cannot bind non-const lvalue reference of type 'double&' to a value of type 'int'
   48 | double& rd3 = i4; // error: type mismatch and reference not const
      |               ^~
p210.cpp:54:13: error: invalid initialization of reference of type 'A&&' from expression of type 'B5'
   54 | A&& rra = f5(); // same as above
      |           ~~^~
p210.cpp: In function 'void enigmatic()':
p210.cpp:69:20: error: binding reference of type 'Banana&&' to 'ConstBanana' {aka 'const Banana'} discards qualifiers
   69 | Banana &&banana1 = ConstBanana(); // error
      |                    ^~~~~~~~~~~~~
p210.cpp:70:20: error: binding reference of type 'Banana&&' to 'const Banana' discards qualifiers
   70 | Banana &&banana2 = Enigma(); // error
      |                    ^~~~~~~~
p210.cpp:65:17: note:   after user-defined conversion: 'Enigma::operator const Banana()'
   65 | struct Enigma { operator const Banana(); };
      |                 ^~~~~~~~
p210.cpp:71:20: error: cannot bind rvalue reference of type 'Banana&&' to lvalue of type 'Banana'
   71 | Banana &&banana3 = Alaska(); // error
      |                    ^~~~~~~~
p210.cpp:66:17: note:   after user-defined conversion: 'Alaska::operator Banana&()'
   66 | struct Alaska { operator Banana&(); };
      |                 ^~~~~~~~
p210.cpp:69:10: warning: unused variable 'banana1' [-Wunused-variable]
   69 | Banana &&banana1 = ConstBanana(); // error
      |          ^~~~~~~
p210.cpp:70:10: warning: unused variable 'banana2' [-Wunused-variable]
   70 | Banana &&banana2 = Enigma(); // error
      |          ^~~~~~~
p210.cpp:71:10: warning: unused variable 'banana3' [-Wunused-variable]
   71 | Banana &&banana3 = Alaska(); // error
      |          ^~~~~~~
p210.cpp: At global scope:
p210.cpp:76:18: error: binding reference of type 'const int&' to 'const volatile int' discards qualifiers
   76 | const int& r26 = cvi; // error: cv-qualifier dropped
      |                  ^~~
p210.cpp:78:17: error: invalid user-defined conversion from 'A6' to 'const int&' [-fpermissive]
   78 | const int& r3 = a6; // error: cv-qualifier dropped
      |                 ^~
p210.cpp:77:13: note: candidate is: 'A6::operator volatile int&()' (near match)
   77 | struct A6 { operator volatile int&(); } a6;
      |             ^~~~~~~~
p210.cpp:77:13: note:   no known conversion from 'volatile int&' to 'const int&'
p210.cpp:78:17: error: conversion from 'volatile int' to 'const int&' discards qualifiers [-fpermissive]
   78 | const int& r3 = a6; // error: cv-qualifier dropped
      |                 ^~
p210.cpp:81:18: error: cannot bind rvalue reference of type 'double&&' to lvalue of type 'double'
   81 | double&& rrd26 = d2; // error: initializer is lvalue of related type
      |                  ^~
p210.cpp:83:14: error: cannot bind rvalue reference of type 'int&&' to lvalue of type 'int'
   83 | int&& rri2 = X6(); // error: result of conversion function is lvalue of related type
      |              ^~~~
p210.cpp:82:13: note:   after user-defined conversion: 'X6::operator int&()'
   82 | struct X6 { operator int&(); };
      |             ^~~~~~~~

検討事項(agenda)

コンパイルエラーの理由を解説する。

<|--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)

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＜この記事は個人の過去の経験に基づく個人の感想です。現在所属する組織、業務とは関係がありません。＞

文書履歴(document history)

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