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次のコードは gcc 4.6 ではコンパイルできますが、4.7 ではコンパイルできません。4.7の問題ですか、それとも4.6の問題ですか? -std=gnu++0x でコンパイル。

#include <utility>

using namespace std;

struct Z {
};

struct X {
    operator Z*() const { return nullptr; }
};

struct Y {
    Y(Z*) {}
};

int main() {
    pair<int, Y> p(make_pair(0, X()));
}

エラー メッセージ:

[hidden]$ g++-mp-4.6 -std=gnu++0x e.cpp 
[hidden]$ g++-mp-4.7 -std=gnu++0x e.cpp 
e.cpp: In function 'int main()':
e.cpp:17:37: error: no matching function for call to 'std::pair<int, Y>::pair(std::pair<int, X>)'
e.cpp:17:37: note: candidates are:
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:204:9: note: template<class ... _Args1, long unsigned int ..._Indexes1, class ... _Args2, long unsigned int ..._Indexes2> std::pair::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>)
/opt/local/include/gcc47/c++/bits/stl_pair.h:204:9: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   'std::pair<int, X>' is not derived from 'std::tuple<_Args1 ...>'
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:153:9: note: template<class ... _Args1, class ... _Args2> std::pair::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>)
/opt/local/include/gcc47/c++/bits/stl_pair.h:153:9: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   cannot convert 'std::make_pair(_T1&&, _T2&&) [with _T1 = int; _T2 = X; typename std::__decay_and_strip<_T2>::__type = X; typename std::__decay_and_strip<_T1>::__type = int]((* & X()))' (type 'std::pair<int, X>') to type 'std::piecewise_construct_t'
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:148:12: note: template<class _U1, class _U2, class> constexpr std::pair::pair(std::pair<_U1, _U2>&&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:148:12: note:   template argument deduction/substitution failed:
/opt/local/include/gcc47/c++/bits/stl_pair.h:145:38: error: no type named 'type' in 'struct std::enable_if<false, void>'
/opt/local/include/gcc47/c++/bits/stl_pair.h:142:12: note: template<class _U1, class _U2, class> constexpr std::pair::pair(_U1&&, _U2&&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:142:12: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   candidate expects 2 arguments, 1 provided
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:136:12: note: template<class _U2, class> constexpr std::pair::pair(const _T1&, _U2&&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:136:12: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   cannot convert 'std::make_pair(_T1&&, _T2&&) [with _T1 = int; _T2 = X; typename std::__decay_and_strip<_T2>::__type = X; typename std::__decay_and_strip<_T1>::__type = int]((* & X()))' (type 'std::pair<int, X>') to type 'const int&'
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:131:12: note: template<class _U1, class> constexpr std::pair::pair(_U1&&, const _T2&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:131:12: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   candidate expects 2 arguments, 1 provided
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:122:7: note: std::pair<_T1, _T2>::pair(std::pair<_T1, _T2>&&) [with _T1 = int; _T2 = Y; std::pair<_T1, _T2> = std::pair<int, Y>]
/opt/local/include/gcc47/c++/bits/stl_pair.h:122:7: note:   no known conversion for argument 1 from 'std::pair<int, X>' to 'std::pair<int, Y>&&'
/opt/local/include/gcc47/c++/bits/stl_pair.h:119:17: note: constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = int; _T2 = Y; std::pair<_T1, _T2> = std::pair<int, Y>]
/opt/local/include/gcc47/c++/bits/stl_pair.h:119:17: note:   no known conversion for argument 1 from 'std::pair<int, X>' to 'const std::pair<int, Y>&'
/opt/local/include/gcc47/c++/bits/stl_pair.h:116:12: note: template<class _U1, class _U2, class> constexpr std::pair::pair(const std::pair<_U1, _U2>&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:116:12: note:   template argument deduction/substitution failed:
/opt/local/include/gcc47/c++/bits/stl_pair.h:113:38: error: no type named 'type' in 'struct std::enable_if<false, void>'
/opt/local/include/gcc47/c++/bits/stl_pair.h:104:26: note: constexpr std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = int; _T2 = Y]
/opt/local/include/gcc47/c++/bits/stl_pair.h:104:26: note:   candidate expects 2 arguments, 1 provided
/opt/local/include/gcc47/c++/bits/stl_pair.h:100:26: note: constexpr std::pair<_T1, _T2>::pair() [with _T1 = int; _T2 = Y]
/opt/local/include/gcc47/c++/bits/stl_pair.h:100:26: note:   candidate expects 0 arguments, 1 provided
4

1 に答える 1

5

それはコンパイルされるべきではありません。

の初期化には、からへp.secondの暗黙の変換が必要です。暗黙的な変換には、最大で1つのユーザー定義の変換のみを含めることができます。必要な変換には2つが必要です。変換演算子を介して、および変換コンストラクターを介して。XYXZ*Z*Y

別のペアからのペア要素の初期化は、暗黙的な変換を介してのみ許可されます。C ++ 11によると：

20.3.2 / 12このコンストラクターは、暗黙的にに変換可能であり、暗黙的にに変換可能でない限り、過負荷解決に参加してconst U&はfirst_typeなりません。const V&second_type

そしてC++98は言った：

20.2.2 / 4引数の対応するメンバーからメンバーを初期化し、必要に応じて暗黙的な変換を実行します。

おそらく、古いバージョンにはこの変換を考慮できるバグがあり、そのバグは新しいバージョンで修正されています。

于 2012-04-20T10:00:24.213 に答える