Item 27: Familiarize yourself with alternatives to overloading on universal references. SlideShare でもちゃんと読めるようにちょっとだけ編集しました。

1. Effective Modern C++
勉強会#7
Item 27
刈谷 満（@kariya_mitsuru）
2015/6/17

2. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
ユニバーサルリファレンスを伴う
オーバーロードの代替案に
慣れ親しもう

3. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
Item 26 で見たように、ユニバーサルリ
ファレンスを伴うオーバーロードはヤバい
でも、オーバーロード便利
（ただし、思った通りに動いてくれれば）

4. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
こんな代替案がある
1. オーバーロードを諦める
2. const T& で渡す
3. 値で渡す
4. タグディスパッチを使う
5. ユニバーサルリファレンスをとるテンプレートに
制約を設ける

5. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
１．オーバーロードを諦める
そのまんま
オーバーロードするから訳が分からなくなる
⇓
オーバーロードしなきゃいんじゃね？

6. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これを
template<typename T>
void logAndAdd(T&& name);
void logAndAdd(int idx);

7. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これを
template<typename T>
void logAndAdd(T&& name);
void logAndAdd(int idx);
こうじゃ
template<typename T>
void logAndAddName(T&& name);
void logAndAddNameIdx(int idx);

8. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これを
template<typename T>
void logAndAdd(T&& name);
void logAndAdd(int idx);
こうじゃ
template<typename T>
void logAndAddName(T&& name);
void logAndAddNameIdx(int idx);
全て解決！

9. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
欠点
コンストラクタには無力
（コンストラクタには名前が無い）
本には名前固定って書いてあるけど、名前無いっすよ…
§12[class.ctor]/p.1 "Constructors do not have names."
てか、やっぱオーバーロード使いたくね？

10. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
２．const T& で渡す
そのまんま
ユニバーサルリファレンス使うから訳がわからなくなる
⇓
ユニバーサルリファレンス使わなきゃいんじゃね？

11. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これを
template<typename T>
void logAndAdd(T&& name);
void logAndAdd(int idx);

12. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これを
template<typename T>
void logAndAdd(T&& name);
void logAndAdd(int idx);
こうじゃ
void logAndAdd(const std::string&
name);
void logAndAdd(int idx);

13. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これを
template<typename T>
void logAndAdd(T&& name);
void logAndAdd(int idx);
こうじゃ
void logAndAdd(const std::string&
name);
void logAndAdd(int idx);
全て解決！

14. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
欠点
ユニバーサルリファレンス使う方法ほど
効率的じゃない
(でもシンプルさは魅力的)
てか、やっぱユニバーサルリファレンス
使いたくね？

15. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
３．値で渡す
そのまんま
…え？効率は？
実はわりと効率がいい（場合も多い）
けど、詳細は Item 41 のお楽しみ

16. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これを
template<typename T>
explicit Person(T&& n)
: name(std::forward<T>(n)) {}
explicit Person(int idx)
: name(nameFromIdx(idx)) {}

17. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これを
template<typename T>
explicit Person(T&& n)
: name(std::forward<T>(n)) {}
explicit Person(int idx)
: name(nameFromIdx(idx)) {}
こうじゃ
explicit Person(std::string n)
: name(std::move(n)) {}
explicit Perosn(int idx)
: name(nameFromIdx(idx)) {}

18. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これを
template<typename T>
explicit Person(T&& n)
: name(std::forward<T>(n)) {}
explicit Person(int idx)
: name(nameFromIdx(idx)) {}
こうじゃ
explicit Person(std::string n)
: name(std::move(n)) {}
explicit Perosn(int idx)
: name(nameFromIdx(idx)) {}
全て解決！

19. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
欠点
この例ならわりと効率いいけど、
ユニバーサルリファレンスほどじゃないし、
ムーブが効率悪い場合には使えない。（std::array とか…）
Item 41 を知らない人が見ると、「効率悪い！」と言われて、
最悪直される（※ 個人の感想です）
0 はともかく、NULL を渡すと int バージョン呼ばれる。
（けど、これに限らないし、Item 8 読め）

20. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
４．タグディスパッチを使う
やっぱ完全転送したい！！！
⇓
みんな大好き、タグディスパッチ

21. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
わ
あ
い
タ
グ
デ
ィ
ス
パ
ッ
チ
め
う
め
う
タ
グ
デ
ィ
ス
パ
ッ
チ
大
好
き
め
う
©芽兎めう

22. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
ユニバーサルリファレンスの問題点
あらゆる引数に完全一致する

23. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
引
数
が
一
つ
し
か
な
い
と
錯
覚
し
て
い
た
？
©藍染惣右介

24. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
解決策
ユニバーサルリファレンス以外
の引数で、オーバーロード解決
を制御すれば良い

25. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
元のコード
std::multiset<std::string> names;
template<typename T>
void logAndAdd(T&& name)
{
auto now = std::chrono::system_clock::now();
log(now, "logAndAdd");
names.emplace(std::forward<T>(name));
}
これに、int を取るオーバーロードを追加したい

26. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
logAndAdd のオーバーロードを追加するんじゃなくて、
logAndAdd 内で実際に処理する関数
logAndAddImpl に処理を委譲する
⇓
この際、T が int の場合用と
そうじゃない場合用の関数を作って
それらの関数に追加の引数を追加して、
それらの型（タグ）が完璧に異なれば良い

27. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
クライアントが呼び出す関数
template<typename T>
void logAndAdd(T&& name)
{
logAndAddImpl(name,
型①か型②);
}
実際に処理する関数
void logAndAddImpl(T&& name,
型①)
{
int 以外の時の処理がここに
}
void logAndAddImpl(int idx,
型②)
{
int の時の処理がここに
}T が int の以外時は「型①」
T が int の時は「型②」
つまり、こんな感じにすれば良い

28. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
クライアントが呼び出す関数
template<typename T>
void logAndAdd(T&& name)
{
logAndAddImpl(name,
型①か型②);
}
実際に処理する関数
void logAndAddImpl(T&& name,
型①)
{
int 以外の時の処理がここに
}
void logAndAddImpl(int idx,
型②)
{
int の時の処理がここに
}T が int の以外時は「型①」
T が int の時は「型②」
つまり、こんな感じにすれば良い
で？
「型①か型②」って？

29. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これが「型①か型②」だ！！！
template<typename T>
void logAndAdd(T&& name)
{
logAndAddImpl(std::forward<T>(name),
std::is_integral<T>());
}
引数だから、() を付
けてオブジェクト生成
T が int 以外の時は「std::false_type」から派生
T が int の時は「std::true_type」から派生

30. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これが「型①か型②」だ！！！
template<typename T>
void logAndAdd(T&& name)
{
logAndAddImpl(std::forward<T>(name),
std::is_integral<T>());
}
引数だから、() を付
けてオブジェクト生成
T が int 以外の時は「std::false_type」から派生
T が int の時は「std::true_type」から派生
でもちょっと
うまくない…

31. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
int i = 10;
…
logAndAdd(i);
template<typename T>
void logAndAdd(T&& name)
{
logAndAddImpl(
std::forward<T>(name),
std::is_integral<T>());
}
うまくない理由
こんな感じで呼び出すと…
T が int& になっちゃう
⇒参照型は整数型じゃないから、
std::false_typeになっちゃう

32. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
これが正しい「型①か型②」だ！！！
template<typename T>
void logAndAdd(T&& name)
{
logAndAddImpl(
std::forward<T>(name),
std::is_integral<typename std::remove_reference<T>::type>());
}
T が int っぽくない時は「std::false_type」
T が int っぽい時は「std::true_type」
T から「参照」を消す

33. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
ちな、処理本体側はこんな感じ
// int 以外用の関数
template<typename T>
void logAndAddImpl(T&& name, std::false_type)
{
auto now = std::chrono::system_clock::now();
log(now, "logAndAdd");
names.emplace(std::forward<T>(name));
}
std::string nameFromIdx(int idx);
// int 用の関数
void logAndAddImpl(int idx, std::true_type)
{
logAndAdd(nameFromIdx(idx));
}
オーバーロード解決の
ためだけにあるから、
パラメータ名は
付けてない
オーバーロード解決の
ためだけにあるから、
パラメータ名は
付けてない
idx から名前に
変換したら、
再度 logAndAdd を
呼び出す

34. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
ちな、処理本体側はこんな感じ
// int 以外用の関数
template<typename T>
void logAndAddImpl(T&& name, std::false_type)
{
auto now = std::chrono::system_clock::now();
log(now, "logAndAdd");
names.emplace(std::forward<T>(name));
}
std::string nameFromIdx(int idx);
// int 用の関数
void logAndAddImpl(int idx, std::true_type)
{
logAndAdd(nameFromIdx(idx));
}
オーバーロード解決の
ためだけにあるから、
パラメータ名は
付けてない
オーバーロード解決の
ためだけにあるから、
パラメータ名は
付けてない
idx から名前に
変換したら、
再度 logAndAdd を
呼び出す
賢いコンパイラなら、
最後のパラメータ使ってないから
消してくれるかも…
こうすれば、ログ出力コード
二重にメンテしなくて済む…

35. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
タグディスパッチSUGEEEE！！！！
タグディスパッチのこと
もっと知りたい！！！
でも EMC++ にはそこまで書かれてない…

36. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
そんなあなたに C++テンプレートテクニック 第2版
あのえぴさんと
アキラさんが
書いてるぞ！！！
Ｃ＋＋１１
完全対応！！！
電子書籍版も
あるぞ！！！
ちょっと読みづらいけど…
タグディスパッチ
だけじゃなくて
テンプレート
全般！！！
まだ全部理解できてないけど…

37. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
タグディスパッチいいよ、タグディスパッチ
もう何もかもタグディスパッチでよくね？

38. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
欠点
コンストラクタには無力
（またっすか…）

39. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
例のダメなクラス
class Person {
public:
template<typename T>
explicit Person(T&& n)
: name(std::forward<Person&>(n)) {}
explicit Person(int idx);
Person(const Person& rhs);
Person(Person&& rhs);
…
};
完全転送
コンストラクタ
int
コンストラクタ コピー
コンストラクタ
ムーブ
コンストラクタ

40. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
例のダメなクラス
class Person {
public:
template<typename T>
explicit Person(T&& n)
: name(std::forward<Person&>(n)) {}
explicit Person(int idx);
Person(const Person& rhs);
Person(Person&& rhs);
…
};
Person p("Nancy");
auto cloneOfP(p);
全部完全転送…
上はいいけど、下に対して
コピーコンストラクタが
呼ばれない…

41. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
５．ユニバーサルリファレンスをとるテンプレートに制約を設ける
そのまんま
ユニバーサルリファレンスを取る関数が
呼ばれて欲しくない時まで呼ばれちゃう
⇓
関数を呼ばれて欲しい時だけ
有効にすればいんじゃね？

42. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
「関数を呼ばれて欲しい時だけ有効に」？
そのためのメタ関数が type_traits に！
std::enable_if
SFINAE と呼ばれる
機構を使ってる

43. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
使い方はこんな感じ
class Person {
public:
template<typename T,
typename = typename std::enable_if<condition>::type>
explicit Person(T&& n);
…
}; この condition の部分に、このコンストラク
タを使っても良い条件式を書く。
ただし、コンパイル時定数のみ！！
関数テンプレートのデフォルトテンプレート引数
（C++11の新機能！）を enable_if にする

44. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
では、今回の場合、どんな時に呼ばれて欲しいのか？
⇓
T が Person じゃない時
T が Person の場合、コピーコンストラクタや
ムーブコンストラクタが呼ばれて欲しいから
そんな時も type_traits

45. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
こんな式
!std::is_same<Person, T>::value
先頭の！に注意 Person と T が
同じ型の時 true
型じゃなくて
bool 値が欲しいから
::value

46. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
class Person {
public:
template<
typename T,
typename = typename std::enable_if<
!std::is_same<Person, T>::value
>::type
>
explicit Person(T&& n);
Person(const Person&);
Person(Person&&);
…
};
ダメでした…
Person p("Nancy");
auto cloneOfP(p);
T は
const char(&)[6]
T は
Person&
p は左辺値だから
T が左辺値参照に…

47. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
T と Person を比較する時には、以下の2点を無視したい
• 参照かどうか
引数の型が Person だろうが Person& だろうが Person&& だろうが、
完全転送コンストラクタじゃ無くてコピー・ムーブコンストラクタが呼ばれてほしい
• const や volatile が付いているかどうか
引数の型が const Person だろうが volatile Person だろうが const volatile
Person だろうが、完全転送コンストラクタじゃ無k（以下略
そんな時も type_traits

48. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
こんな式
!std::is_same<Person,
typename std::decay<T>::type
>::value
ホントはこれだと、配列や関数をポインタに変換しちゃうけど、今回の場合は問題なし
参照を取ってから、
更に cv 修飾を取る！！
値じゃなくて型が
欲しいから ::type

49. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
class Person {
public:
template<
typename T,
typename = typename std::enable_if<
!std::is_same<
Person,
typename std::decay<T>::type
>::value
>::type
>
explicit Person(T&& n);
Person(const Person&);
Person(Person&&);
…
};
まだダメでした…
class SP : public Person {
public:
SP(const SP& rhs)
: Person(rhs)
{ … }
SP(SP&& rhs)
: Person(std::move(rhs))
{ … }
…
};
T は
const SP&
T は
SP&&

50. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
完全転送コンストラクタは、
T が Person かその派生クラスの時には
無効化されて欲しい
そんな時も type_traits

51. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
class Person {
public:
template<
typename T,
typename = typename std::enable_if<
!std::is_base_of<Person,
typename std::decay<T>::type
>::value
>::type
>
explicit Person(T&& n);
…
};
typename std::decay<T>::type が
Person の派生クラスの時 true
Person そのものでもＯＫ！
だいぶ長い…

52. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
class Person {
public:
template<
typename T,
typename = typename std::enable_if<
!std::is_base_of<Person,
typename std::decay<T>::type
>::value
>::type
>
explicit Person(T&& n);
…
};
typename std::decay<T>::type が
Person の派生クラスの時 true
Person そのものでもＯＫ！
だいぶ長い…
Errata に補足あり！
std::is_base_of<int, int> は
false になるよ！

53. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
class Person {
public:
template<
typename T,
typename = std::enable_if_t<
!std::is_base_of<Person,
std::decay_t<T>
>::value
>
>
explicit Person(T&& n);
…
};
ちなみに C++14 だったら
もうちょっとスマート（かな？）
typename と ::type が
必要ない！
C++14 でも ::value は必要
（大人の事情により _v は入らず…）

54. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
でもまだ int を引数にとる
コンストラクタが追加されてない…

55. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
class Person {
public:
template<typename T,
typename = std::enable_if_t<
!std::is_base_of<Person, std::decay_t<T>>::value
&&
!std::is_integral<std::remove_reference_t<T>>::value
>
>
explicit Person(T&& n)
: name(std::forward<T>(n))
{ … }
explicit Person(int idx)
: name(nameFromIdx(idx))
{ … }
…
private:
std::string name;
};
C++14 でも
超絶長い…

56. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
SFINAE SUGEEEE！！！！
SFINAEのこと
もっと知りたい！！！
でも EMC++ にはそこまで書かれてない…

57. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
そんなあなたに C++テンプレートテクニック 第2版
あのえぴさんと
アキラさんが
書いてるぞ！！！
Ｃ＋＋１１
完全対応！！！
電子書籍版も
あるぞ！！！
ちょっと読みづらいけど…
SFINAE
だけじゃなくて
テンプレート
全般！！！
まだ全部理解できてないけど…

58. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
トレードオフ
1. オーバーロードを諦める
2. const T& で渡す
3. 値で渡す
4. タグディスパッチを使う
5. ユニバーサルリファレンスをとる
テンプレートに制約を設ける
完全転送を
諦める
（シンプル）
完全転送を
使う
（効率いい）

59. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
完全転送の欠点
1.完全転送できない場合がある
（完全じゃね～し… Item 30を参照）
2.クライアントが誤った引数を渡した時、
とち狂ったエラーメッセージが出る

60. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
とち狂ったメッセージの例
例えば、こんな感じに間違った場合…
Person p(u"Konrad Zuse");
巷で話題の
utf-16 リテラル

61. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
prog.cc: In instantiation of 'Person::Person(T&&) [with T = const char32_t (&)[5]; <template-parameter-1-2> = void]':
prog.cc:30:18: required from here
prog.cc:16:27: error: no matching function for call to 'std::__cxx11::basic_string<char>::basic_string(const char32_t [5])'
: name(std::forward<T>(n))
^
In file included from /usr/local/gcc-head/include/c++/6.0.0/string:52:0,
from prog.cc:2:
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:535:9: note: candidate: template<class _InputIterator, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(_InputIterator, _InputIterator, const _Alloc&)
basic_string(_InputIterator __beg, _InputIterator __end,
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:535:9: note: template argument deduction/substitution failed:
prog.cc:16:27: note: candidate expects 3 arguments, 1 provided
: name(std::forward<T>(n))
^
In file included from /usr/local/gcc-head/include/c++/6.0.0/string:52:0,
from prog.cc:2:
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:512:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc =
std::allocator<char>]
basic_string(basic_string&& __str, const _Alloc& __a)
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:512:7: note: candidate expects 2 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:508:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc =
std::allocator<char>]
basic_string(const basic_string& __str, const _Alloc& __a)
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:508:7: note: candidate expects 2 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:504:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::initializer_list<_Tp>, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:504:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'std::initializer_list<char>'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:477:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(basic_string&& __str) noexcept
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:477:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'std::__cxx11::basic_string<char>&&'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:465:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type, _CharT, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>;
_Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(size_type __n, _CharT __c, const _Alloc& __a = _Alloc())
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:465:7: note: candidate expects 3 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:455:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:455:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'const char*'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:445:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type, const _Alloc&) [with _CharT = char; _Traits =
std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(const _CharT* __s, size_type __n,
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:445:7: note: candidate expects 3 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:427:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type,
std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(const basic_string& __str, size_type __pos,
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:427:7: note: candidate expects 4 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:411:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type,
std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(const basic_string& __str, size_type __pos,
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:411:7: note: candidate expects 3 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:399:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(const basic_string& __str)
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:399:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'const std::__cxx11::basic_string<char>&'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:391:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(const _Alloc& __a)
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:391:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'const std::allocator<char>&'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:380:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string() [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string()
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:380:7: note: candidate expects 0 arguments, 1 provided
GCC さん

62. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
prog.cc:16:4: error: no matching constructor for initialization of 'std::string' (aka 'basic_string<char, char_traits<char>, allocator<char> >')
: name(std::forward<T>(n))
^ ~~~~~~~~~~~~~~~~~~
prog.cc:30:9: note: in instantiation of function template specialization 'Person::Person<char32_t const (&)[5], void>' requested here
Person p(U"name");
^
/usr/local/libcxx-head/include/c++/v1/string:1326:40: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'const allocator_type' (aka 'const std::__1::allocator<char>') for 1st argument
_LIBCPP_INLINE_VISIBILITY explicit basic_string(const allocator_type& __a)
^
/usr/local/libcxx-head/include/c++/v1/string:1333:5: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'const std::__1::basic_string<char>' for 1st argument
basic_string(const basic_string& __str);
^
/usr/local/libcxx-head/include/c++/v1/string:1338:5: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'std::__1::basic_string<char>' for 1st argument
basic_string(basic_string&& __str)
^
/usr/local/libcxx-head/include/c++/v1/string:1348:31: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'const value_type *' (aka 'const char *') for 1st argument
_LIBCPP_INLINE_VISIBILITY basic_string(const value_type* __s);
^
/usr/local/libcxx-head/include/c++/v1/string:1369:5: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'initializer_list<value_type>' for 1st argument
basic_string(initializer_list<value_type> __il);
^
/usr/local/libcxx-head/include/c++/v1/string:1363:9: note: candidate constructor template not viable: requires 2 arguments, but 1 was provided
basic_string(_InputIterator __first, _InputIterator __last);
^
/usr/local/libcxx-head/include/c++/v1/string:1366:9: note: candidate constructor template not viable: requires 3 arguments, but 1 was provided
basic_string(_InputIterator __first, _InputIterator __last, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1323:31: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
_LIBCPP_INLINE_VISIBILITY basic_string()
^
/usr/local/libcxx-head/include/c++/v1/string:1346:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(basic_string&& __str, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1334:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(const basic_string& __str, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1350:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(const value_type* __s, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1352:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(const value_type* __s, size_type __n);
^
/usr/local/libcxx-head/include/c++/v1/string:1356:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(size_type __n, value_type __c);
^
/usr/local/libcxx-head/include/c++/v1/string:1371:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(initializer_list<value_type> __il, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1354:5: note: candidate constructor not viable: requires 3 arguments, but 1 was provided
basic_string(const value_type* __s, size_type __n, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1358:5: note: candidate constructor not viable: requires 3 arguments, but 1 was provided
basic_string(size_type __n, value_type __c, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1359:5: note: candidate constructor not viable: requires at least 2 arguments, but 1 was provided
basic_string(const basic_string& __str, size_type __pos, size_type __n = npos,
^
Clang さん

63. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
この例の場合、
ユニバーサルリファレンスの引数は
std::string の初期化に使われることが
分かっているので、
static_assert でチェックすることはできる。

64. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
class Person {
public:
template<typename T,
typename = std::enable_if_t<
!std::is_base_of<Person, std::decay_t<T>>::value
&&
!std::is_integral<std::remove_reference_t<T>>::value
>
>
explicit Person(T&& n)
: name(std::forward<T>(n))
{
// assert that a std::string can be created from a T object
static_assert(
std::is_constructible<std::string, T>::value,
"Parameter n can't be used to construct a std::string"
);
…
}
…
};
こんな感じでチェック！

65. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
prog.cc: In instantiation of 'Person::Person(T&&) [with T = const char32_t (&)[5]; <template-parameter-1-2> = void]':
prog.cc:35:18: required from here
prog.cc:16:27: error: no matching function for call to 'std::__cxx11::basic_string<char>::basic_string(const char32_t [5])'
: name(std::forward<T>(n))
^
In file included from /usr/local/gcc-head/include/c++/6.0.0/string:52:0,
from prog.cc:2:
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:535:9: note: candidate: template<class _InputIterator, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(_InputIterator, _InputIterator, const _Alloc&)
basic_string(_InputIterator __beg, _InputIterator __end,
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:535:9: note: template argument deduction/substitution failed:
prog.cc:16:27: note: candidate expects 3 arguments, 1 provided
: name(std::forward<T>(n))
^
In file included from /usr/local/gcc-head/include/c++/6.0.0/string:52:0,
from prog.cc:2:
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:512:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc =
std::allocator<char>]
basic_string(basic_string&& __str, const _Alloc& __a)
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:512:7: note: candidate expects 2 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:508:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc =
std::allocator<char>]
basic_string(const basic_string& __str, const _Alloc& __a)
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:508:7: note: candidate expects 2 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:504:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::initializer_list<_Tp>, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:504:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'std::initializer_list<char>'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:477:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(basic_string&& __str) noexcept
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:477:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'std::__cxx11::basic_string<char>&&'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:465:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type, _CharT, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>;
_Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(size_type __n, _CharT __c, const _Alloc& __a = _Alloc())
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:465:7: note: candidate expects 3 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:455:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:455:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'const char*'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:445:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type, const _Alloc&) [with _CharT = char; _Traits =
std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(const _CharT* __s, size_type __n,
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:445:7: note: candidate expects 3 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:427:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type,
std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(const basic_string& __str, size_type __pos,
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:427:7: note: candidate expects 4 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:411:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type,
std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(const basic_string& __str, size_type __pos,
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:411:7: note: candidate expects 3 arguments, 1 provided
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:399:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(const basic_string& __str)
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:399:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'const std::__cxx11::basic_string<char>&'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:391:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string(const _Alloc& __a)
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:391:7: note: no known conversion for argument 1 from 'const char32_t [5]' to 'const std::allocator<char>&'
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:380:7: note: candidate: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string() [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string()
^
/usr/local/gcc-head/include/c++/6.0.0/bits/basic_string.h:380:7: note: candidate expects 0 arguments, 1 provided
prog.cc:19:3: error: static assertion failed: Parameter n can't be used to construct a std::string
static_assert(
^
GCC さん
ここ！

66. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
prog.cc:16:4: error: no matching constructor for initialization of 'std::string' (aka 'basic_string<char, char_traits<char>, allocator<char> >')
: name(std::forward<T>(n))
^ ~~~~~~~~~~~~~~~~~~
prog.cc:35:9: note: in instantiation of function template specialization 'Person::Person<char32_t const (&)[5], void>' requested here
Person p(U"name");
^
/usr/local/libcxx-head/include/c++/v1/string:1326:40: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'const allocator_type' (aka 'const std::__1::allocator<char>') for 1st argument
_LIBCPP_INLINE_VISIBILITY explicit basic_string(const allocator_type& __a)
^
/usr/local/libcxx-head/include/c++/v1/string:1333:5: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'const std::__1::basic_string<char>' for 1st argument
basic_string(const basic_string& __str);
^
/usr/local/libcxx-head/include/c++/v1/string:1338:5: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'std::__1::basic_string<char>' for 1st argument
basic_string(basic_string&& __str)
^
/usr/local/libcxx-head/include/c++/v1/string:1348:31: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'const value_type *' (aka 'const char *') for 1st argument
_LIBCPP_INLINE_VISIBILITY basic_string(const value_type* __s);
^
/usr/local/libcxx-head/include/c++/v1/string:1369:5: note: candidate constructor not viable: no known conversion from 'char32_t const[5]' to 'initializer_list<value_type>' for 1st argument
basic_string(initializer_list<value_type> __il);
^
/usr/local/libcxx-head/include/c++/v1/string:1363:9: note: candidate constructor template not viable: requires 2 arguments, but 1 was provided
basic_string(_InputIterator __first, _InputIterator __last);
^
/usr/local/libcxx-head/include/c++/v1/string:1366:9: note: candidate constructor template not viable: requires 3 arguments, but 1 was provided
basic_string(_InputIterator __first, _InputIterator __last, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1323:31: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
_LIBCPP_INLINE_VISIBILITY basic_string()
^
/usr/local/libcxx-head/include/c++/v1/string:1346:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(basic_string&& __str, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1334:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(const basic_string& __str, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1350:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(const value_type* __s, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1352:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(const value_type* __s, size_type __n);
^
/usr/local/libcxx-head/include/c++/v1/string:1356:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(size_type __n, value_type __c);
^
/usr/local/libcxx-head/include/c++/v1/string:1371:5: note: candidate constructor not viable: requires 2 arguments, but 1 was provided
basic_string(initializer_list<value_type> __il, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1354:5: note: candidate constructor not viable: requires 3 arguments, but 1 was provided
basic_string(const value_type* __s, size_type __n, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1358:5: note: candidate constructor not viable: requires 3 arguments, but 1 was provided
basic_string(size_type __n, value_type __c, const allocator_type& __a);
^
/usr/local/libcxx-head/include/c++/v1/string:1359:5: note: candidate constructor not viable: requires at least 2 arguments, but 1 was provided
basic_string(const basic_string& __str, size_type __pos, size_type __n = npos,
^
prog.cc:19:3: error: static_assert failed "Parameter n can't be used to construct a std::string"
static_assert(
^
Clang さん
ここ！

67. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
でも、今回の例の場合、
ホントはそもそも std::enable_if の条件に
std::is_constructible 使えばいいと
思うんだけど…
Errata には、コンパイラのエラーメッセージが、
「そんなコンストラクタ無いよ」ってなるので、
意味が変わっちゃうって書いてあるけど…

68. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
こんな感じ
class Person {
public:
template<typename T,
typename = std::enable_if_t<std::is_constructible<std::string, T>::value>
>
explicit Person(T&& n)
: name(std::forward<T>(n))
{
…
}
…
};
こんな感じで無効化！

69. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
prog.cc: In function 'int main()':
prog.cc:26:18: error: no matching function for call to 'Person::Person(const char32_t [5])'
Person p(U"name");
^
prog.cc:18:2: note: candidate: Person::Person(Person&&)
Person(Person&&) = default;
^
prog.cc:18:2: note: no known conversion for argument 1 from 'const char32_t [5]' to
'Person&&'
prog.cc:17:2: note: candidate: Person::Person(const Person&)
Person(const Person&) = default;
^
prog.cc:17:2: note: no known conversion for argument 1 from 'const char32_t [5]' to 'const
Person&'
prog.cc:11:11: note: candidate: template<class T, class> Person::Person(T&&)
explicit Person(T&& n)
^
prog.cc:11:11: note: template argument deduction/substitution failed:
GCC さん

70. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
prog.cc:26:9: error: no matching constructor for initialization of 'Person'
Person p(U"name");
^ ~~~~~~~
prog.cc:17:2: note: candidate constructor not viable: no known conversion from 'const
char32_t [5]' to 'const Person' for 1st argument
Person(const Person&) = default;
^
prog.cc:18:2: note: candidate constructor not viable: no known conversion from 'const
char32_t [5]' to 'Person' for 1st argument
Person(Person&&) = default;
^
/usr/local/libcxx-head/include/c++/v1/type_traits:244:78: note: candidate template ignored:
disabled by 'enable_if' [with T = char32_t const (&)[5]]
template <bool _Bp, class _Tp = void> using enable_if_t = typename enable_if<_Bp,
_Tp>::type;
^
Clang さん
Clang さんすごい！

71. Item 27: Familiarize yourself with alternatives
to overloading on universal references.
覚えておくこと
• ユニバーサルリファレンスとオーバーロードの組み合わせの代替案
には、異なる関数名を使用する、パラメータをconstへの左辺値参
照で渡す、パラメータを値で渡す、タグディスパッチを使う、が含まれ
る。
• std::enable_if を通じてテンプレートを制約することで、ユニ
バーサルリファレンスとオーバーロードを同時に使用できるが、それ
はコンパイラがユニバーサルリファレンスのオーバーロードを使用す
る条件を制御する。
• ユニバーサルリファレンスはたいてい効率上の利点があるが、概し
て使い勝手の点で欠点もある。