std::tuple_cat次のように、引数リストのサブセット選択をタプルに使用します。
template <class...>
struct odds;
template <class T1>
struct odds<T1>
{
typedef std::tuple<T1> type;
static type value(T1&& t1)
{
return std::make_tuple(std::forward<T1>(t1));
}
};
template <class T1, class T2>
struct odds<T1, T2>
{
typedef std::tuple<T1> type;
static type value(T1&& t1, T2&&)
{
return std::make_tuple(std::forward<T1>(t1));
}
};
template <class T1, class T2, class... TTail>
struct odds<T1, T2, TTail...>
{
typedef decltype(std::tuple_cat(T1(), typename odds<TTail...>::type())) type; // L32
static type value(T1&& t1, T2&&, TTail&&... rest)
{
return std::tuple_cat(std::forward<T1>(t1), odds<TTail...>::value(std::forward<TTail>(rest)...)); // L35
}
};
、テストケースとして以下を使用：
// assume <tuple>, <utility> are included at top of file
template <class... T>
auto foo(T... x) -> typename odds<T...>::type
{
return odds<T...>::value(x...);
//...
}
int main() {
auto bar = foo(5, true, 6, false); // L46
auto baz = odds<int, bool, int, bool>::value(5, true, 6, false); // L47
// bar, baz should be tuple<int,int> with value { 5, 6 }
}
ただし、clang-3.1 と gcc-4.7.2 の両方でテンプレート推定の問題が発生します。
クラン出力:
test.cc:32:19: error: no matching function for call to 'tuple_cat'
typedef decltype(std::tuple_cat(T1(), typename odds<TTail...>::type())) type;
^~~~~~~~~~~~~~
test.cc:40:30: note: in instantiation of template class 'odds<int, bool, int, bool>' requested here
auto foo(T... x) -> typename odds<T...>::type
^
test.cc:40:6: note: while substituting deduced template arguments into function template 'foo' [with T = <int, bool, int, bool>]
auto foo(T... x) -> typename odds<T...>::type
^
/usr/include/c++/v1/tuple:1063:1: note: candidate template ignored: substitution failure [with _Tuple0 = int, _Tuples = <std::__1::tuple<int>>]
tuple_cat(_Tuple0&& __t0, _Tuples&&... __tpls)
^
/usr/include/c++/v1/tuple:987:1: note: candidate function not viable: requires 0 arguments, but 2 were provided
tuple_cat()
^
test.cc:46:13: error: no matching function for call to 'foo'
auto bar = foo(5, true, 6, false);
^~~
test.cc:40:6: note: candidate template ignored: substitution failure [with T = <int, bool, int, bool>]
auto foo(T... x) -> typename odds<T...>::type
^
test.cc:35:10: error: no matching function for call to 'tuple_cat'
return std::tuple_cat(std::forward<T1>(t1), odds<TTail...>::value(std::forward<TTail>(rest)...));
^~~~~~~~~~~~~~
test.cc:47:41: note: in instantiation of member function 'odds<int, bool, int, bool>::value' requested here
auto baz = odds<int, bool, int, bool>::value(5,true,6,false);
^
/usr/include/c++/v1/tuple:1063:1: note: candidate template ignored: substitution failure [with _Tuple0 = int, _Tuples = <std::__1::tuple<int>>]
tuple_cat(_Tuple0&& __t0, _Tuples&&... __tpls)
^
/usr/include/c++/v1/tuple:987:1: note: candidate function not viable: requires 0 arguments, but 2 were provided
tuple_cat()
^
3 errors generated.
Gcc 出力:
test.cc: In instantiation of ‘struct odds<int, bool, int, bool>’:
test.cc:40:6: required by substitution of ‘template<class ... T> typename odds<T ...>::type foo(T ...) [with T = {int, bool, int, bool}]’
test.cc:46:34: required from here
test.cc:32:74: error: no matching function for call to ‘tuple_cat(int, odds<int, bool>::type)’
test.cc:32:74: note: candidate is:
In file included from test.cc:1:0:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.2/include/g++-v4/tuple:1027:5: note: template<class ... _Tpls, class> constexpr typename std::__tuple_cat_result<_Tpls ...>::__type std::tuple_cat(_Tpls&& ...)
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.2/include/g++-v4/tuple:1027:5: note: template argument deduction/substitution failed:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.2/include/g++-v4/tuple:1024:31: error: no type named ‘type’ in ‘struct std::enable_if<false, void>’
test.cc: In function ‘int main()’:
test.cc:46:34: error: no matching function for call to ‘foo(int, bool, int, bool)’
test.cc:46:34: note: candidate is:
test.cc:40:6: note: template<class ... T> typename odds<T ...>::type foo(T ...)
test.cc:40:6: note: substitution of deduced template arguments resulted in errors seen above
test.cc:46:34: error: unable to deduce ‘auto’ from ‘<expression error>’
test.cc:47:13: error: ‘value’ is not a member of ‘odds<int, bool, int, bool>’
test.cc:47:61: error: unable to deduce ‘auto’ from ‘<expression error>’
ここでは Gcc がもう少し役に立ちます。特にエラーが発生した場合は
test.cc:32:74: error: no matching function for call to ‘tuple_cat(int, odds<int, bool>::type)’
目標は、引数を再帰的にアンパックし、選択を収集タプルに収集して返す関数を呼び出すことです。フラットな方法で蓄積するためにstd::tuple_cat()、再帰的な末尾のタプルをフラット化し、ヘッドを追加して、タプルを返すために使用しています。転送は、再帰中に参照修飾子を削除しないために使用されます。
コードの後半で、結果のタプルがアンパックされて別の可変引数関数が呼び出されますが、それはこのエラーの範囲外です。
明らかに、微妙ではあるが重要な詳細をどこかで省略しましたが、根本的な問題を追跡するのが非常に困難であることに気付きました。