C++STLのarrayの型にpair型を入れるとコンパイルエラーが発生する

Question

前提

C++STLのarrayとpairの文法についての質問です．

実現したいこと

arrayの型にpair型を与えるとコンパイルエラーが起きる理由について知りたいです．

発生している問題・エラーメッセージ

Main.cpp:6:10: error: no viable conversion from 'int' to 'std::pair<int, std::array<int, 2> >'
        {1, {1, 2}}, 
         ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:303:17: note: candidate constructor not viable: no known conversion from 'int' to 'const std::pair<int, std::array<int, 2> > &' for 1st argument
      constexpr pair(const pair&) = default;
                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:304:17: note: candidate constructor not viable: no known conversion from 'int' to 'std::pair<int, std::array<int, 2> > &&' for 1st argument
      constexpr pair(pair&&) = default;
                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:291:19: note: candidate template ignored: could not match 'pair<type-parameter-0-0, type-parameter-0-1>' against 'int'
        constexpr pair(const pair<_U1, _U2>& __p)
                  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:360:12: note: candidate template ignored: could not match 'pair<type-parameter-0-0, type-parameter-0-1>' against 'int'
        constexpr pair(pair<_U1, _U2>&& __p)
                  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:300:21: note: explicit constructor is not a candidate
        explicit constexpr pair(const pair<_U1, _U2>& __p)
                           ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:370:21: note: explicit constructor is not a candidate
        explicit constexpr pair(pair<_U1, _U2>&& __p)
                           ^
Main.cpp:6:13: error: no matching constructor for initialization of 'std::pair<int, std::array<int, 2> >'
        {1, {1, 2}}, 
            ^~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:260:17: note: candidate constructor template not viable: no known conversion from 'int' to 'const std::array<int, 2>' for 2nd argument
      constexpr pair(const _T1& __a, const _T2& __b)
                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:311:18: note: candidate constructor template not viable: no known conversion from 'int' to 'const std::array<int, 2>' for 2nd argument
       constexpr pair(_U1&& __x, const _T2& __y)
                 ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:269:26: note: candidate template ignored: requirement '!_PCC<true, int, std::array<int, 2> >::_ImplicitlyConvertiblePair()' was not satisfied [with _U1 = int, _U2 = std::array<int, 2>]
      explicit constexpr pair(const _T1& __a, const _T2& __b)
                         ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:318:27: note: candidate template ignored: requirement '_PCC<true, int, std::array<int, 2> >::_MoveCopyPair()' was not satisfied [with _U1 = int]
       explicit constexpr pair(_U1&& __x, const _T2& __y)
                          ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:325:18: note: candidate template ignored: requirement '_PCC<true, int, std::array<int, 2> >::_CopyMovePair()' was not satisfied [with _U2 = int]
       constexpr pair(const _T1& __x, _U2&& __y)
                 ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:332:17: note: candidate template ignored: requirement '_PCC<true, int, std::array<int, 2> >::_CopyMovePair()' was not satisfied [with _U2 = int]
       explicit pair(const _T1& __x, _U2&& __y)
                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:341:12: note: candidate template ignored: requirement '_PCC<true, int, std::array<int, 2> >::_MoveConstructiblePair()' was not satisfied [with _U1 = int, _U2 = int]
        constexpr pair(_U1&& __x, _U2&& __y)
                  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:350:21: note: candidate template ignored: requirement '_PCC<true, int, std::array<int, 2> >::_MoveConstructiblePair()' was not satisfied [with _U1 = int, _U2 = int]
        explicit constexpr pair(_U1&& __x, _U2&& __y)
                           ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:291:19: note: candidate constructor template not viable: requires single argument '__p', but 2 arguments were provided
        constexpr pair(const pair<_U1, _U2>& __p)
                  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:300:21: note: candidate constructor template not viable: requires single argument '__p', but 2 arguments were provided
        explicit constexpr pair(const pair<_U1, _U2>& __p)
                           ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:303:17: note: candidate constructor not viable: requires 1 argument, but 2 were provided
      constexpr pair(const pair&) = default;
                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:304:17: note: candidate constructor not viable: requires 1 argument, but 2 were provided
      constexpr pair(pair&&) = default;
                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:360:12: note: candidate constructor template not viable: requires single argument '__p', but 2 arguments were provided
        constexpr pair(pair<_U1, _U2>&& __p)
                  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:370:21: note: candidate constructor template not viable: requires single argument '__p', but 2 arguments were provided
        explicit constexpr pair(pair<_U1, _U2>&& __p)
                           ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:375:9: note: candidate constructor template not viable: requires 3 arguments, but 2 were provided
        pair(piecewise_construct_t, tuple<_Args1...>, tuple<_Args2...>);
        ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:229:26: note: candidate constructor template not viable: requires 0 arguments, but 2 were provided
      _GLIBCXX_CONSTEXPR pair()
                         ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:242:26: note: candidate constructor template not viable: requires 0 arguments, but 2 were provided
      explicit constexpr pair()
                         ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:436:9: note: candidate constructor template not viable: requires 4 arguments, but 2 were provided
        pair(tuple<_Args1...>&, tuple<_Args2...>&,
        ^
2 errors generated.

該当のソースコード

#include <bits/stdc++.h>
using namespace std;
int main(void){
    // Your code here!
    array<pair<int, array<int, 2>>, 2> {
        {1, {1, 2}}, 
        {2, {1, 2}}
    };
    return 0;
}

試したこと

vectorに直すとコンパイルが通るようになります．

#include <bits/stdc++.h>
using namespace std;
int main(void){
    // Your code here!
    vector<pair<int, array<int, 2>>> {
        {1, {1, 2}}, 
        {2, {1, 2}}
    };
    return 0;
}

補足情報（FW/ツールのバージョンなど）

C17++ / clang version 10.0.0-4ubuntu1
https://paiza.io/projects/oufhnDISoQkzfmbs9JLyng?language=cpp

Answer 1

std::array は集成体として扱われるルールなので、波括弧の中をコンストラクタに渡す std::vector とは解釈が異なります。 中括弧が余計にひとつ必要です。

array<pair<int, array<int, 2>>, 2> {{
    {1, {1, 2}}, 
    {2, {1, 2}}
}};

Comments

[moosan]

理解・解決致しました．
回答ありがとうございます．