【C++】error: no matching function for call to ‘max(int&, std::map<int, int>::size_type)’の対処法

症状

AtCoder Beginner Contest 397のC問題を実施していた時に以下のエラーメッセージが表示されました。 エラーメッセージを翻訳すると「エラー: ‘max(int&, std::map::size_type)’ の呼び出しに一致する関数がありません」でした

error

Main.cpp: In function ‘int main()’:
Main.cpp:33:14: error: no matching function for call to ‘max(int&, std::map<int, int>::size_type)’
   33 |     ans = max(ans,before.size() + after.size());
      |           ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/12/bits/specfun.h:45,
                 from /usr/include/c++/12/cmath:1935,
                 from /usr/include/x86_64-linux-gnu/c++/12/bits/stdc++.h:41,
                 from Main.cpp:1:
/usr/include/c++/12/bits/stl_algobase.h:254:5: note: candidate: ‘template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)’
  254 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/12/bits/stl_algobase.h:254:5: note:   template argument deduction/substitution failed:
Main.cpp:33:14: note:   deduced conflicting types for parameter ‘const _Tp’ (‘int’ and ‘std::map<int, int>::size_type’ {aka ‘long unsigned int’})
   33 |     ans = max(ans,before.size() + after.size());
      |           ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/12/bits/stl_algobase.h:300:5: note: candidate: ‘template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)’
  300 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/12/bits/stl_algobase.h:300:5: note:   template argument deduction/substitution failed:
Main.cpp:33:14: note:   deduced conflicting types for parameter ‘const _Tp’ (‘int’ and ‘std::map<int, int>::size_type’ {aka ‘long unsigned int’})
   33 |     ans = max(ans,before.size() + after.size());
      |           ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/12/algorithm:61,
                 from /usr/include/x86_64-linux-gnu/c++/12/bits/stdc++.h:65:
/usr/include/c++/12/bits/stl_algo.h:5746:5: note: candidate: ‘template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)’
 5746 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/12/bits/stl_algo.h:5746:5: note:   template argument deduction/substitution failed:
Main.cpp:33:14: note:   mismatched types ‘std::initializer_list<_Tp>’ and ‘int’
   33 |     ans = max(ans,before.size() + after.size());
      |           ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/12/bits/stl_algo.h:5756:5: note: candidate: ‘template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)’
 5756 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/12/bits/stl_algo.h:5756:5: note:   template argument deduction/substitution failed:
Main.cpp:33:14: note:   mismatched types ‘std::initializer_list<_Tp>’ and ‘int’
   33 |     ans = max(ans,before.size() + after.size());
      |           ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

code

#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0;i < (n);i++)

int main() {
  int ans = 0;
  int n = 0;
  cin >> n;
  vector<int> a(n);

  for(auto& x :a)cin >> x;

  // 分割ロジック
  for(int i = 1;i < n-1;i++){
    map<int,int> before;
    map<int,int> after;

    // beforeCountロジック
    for(int j = 0;j < i;j++){
      if(!(before.count(a.at(j)))){
        before[a.at(j)] = 1;
      }
    }

    // afterCountロジック
    for(int j = i;j < n-1;j++){
      if(!(after.count(a.at(j)))){
        after[a.at(j)] = 1;
      }
    }
  
    //最大値を設定する
    ans = max(ans,before.size() + after.size());
  }
  
  cout << ans << endl;
  return 0;
}

解決策

maxでの引数がintで揃っていなかったため、エラーが表示されていたようなので、intの変数を作ってそこにmap.size()を入れることで、解決しました(なお、ロジックはおかしい模様)

code

#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0;i < (n);i++)

int main() {
  int ans = 0;
  int n = 0;
  cin >> n;
  vector<int> a(n);

  for(auto& x :a)cin >> x;

  // 分割ロジック
  for(int i = 1;i < n-1;i++){
    map<int,int> before;
    map<int,int> after;

    // beforeCountロジック
    for(int j = 0;j < i;j++){
      if(!(before.count(a.at(j)))){
        before[a.at(j)] = 1;
      }
    }

    // afterCountロジック
    for(int j = i;j < n-1;j++){
      if(!(after.count(a.at(j)))){
        after[a.at(j)] = 1;
      }
    }
    //ここに2行追加ｓ
    int beforeSize = before.size();
    int afterSize = after.size();
    //最大値を設定する
    ans = max(ans,beforeSize + afterSize);
  }
  
  cout << ans << endl;
  return 0;
}

参考