Language: C++

 #include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <cassert>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
#include <iomanip>
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define irep(it, stl) for(auto it = stl.begin(); it != stl.end(); it++)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout << (ans) << endl
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
int dx8[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy8[8] = {0, 1, 0, -1, 1, 1, -1, -1};
int dx4[4] = {1, 0, -1, 0};
int dy4[4] = {0, 1, 0, -1};
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;} return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;} return 0;}
template<typename A, size_t N, typename T> void Fill(A (&array)[N], const T &val){ fill((T*)array, (T*)(array+N), val); }
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);}};
const int INF = INT_MAX;
const ll LLINF = 1LL<<60;
const ll MOD = 1000000007;
const double EPS = 1e-9;
template < typename T > inline string toString( const T &a ) { ostringstream oss; oss << a; return oss.str(); };



int H,W;
void dfs(int i,int j,int direction,int& count,vs& S){
        int curX = i,curY =j;
        //同じ方向にしか行けないbfs
        //0だったら全方向、東から1,2,3,4
        if(direction == 0){
            for(int i=1;i<=4;i++){
                int nX = curX + dx4[i-1],nY = curY + dy4[i-1];
                if(nX < 0 || nX > W || nY < 0 || nY >H || S[nX][nY] == '#') continue;
                     dfs(nX,nY,i,count,S);
            }                          
        }else{
                int nX = curX + dx4[i-1],nY = curY + dy4[i-1];
                if(nX < 0 || nX > W || nY < 0 || nY >H || S[nX][nY] == '#'){
                    return;
                };
                (*count)++;
                dfs(nX,nY,direction,count,S);
        }      
    return;
}

int main()
{
  cin >> H >> W;
  vs S(H);
  rep(i,H){
      cin >> S[i];
  }
  int ans=0;
  int count = 0;
  rep(i,H){
      rep(j,W){
          if(S[i][j] == '#') continue;
          dfs(i,j,0,&count,S);
          ans = max(ans,count);
          count = 0;
      }
  }
  fin(ans);
}



Compiler output:
a.cc: In function 'void dfs(int, int, int, int&, vs&)':
a.cc:80:18: error: invalid type argument of unary '*' (have 'int')
   80 |                 (*count)++;
      |                  ^~~~~~
a.cc: In function 'int main()':
a.cc:98:21: error: invalid conversion from 'int*' to 'int' [-fpermissive]
   98 |           dfs(i,j,0,&count,S);
      |                     ^~~~~~
      |                     |
      |                     int*
a.cc:65:41: note:   initializing argument 4 of 'void dfs(int, int, int, int&, vs&)'
   65 | void dfs(int i,int j,int direction,int& count,vs& S){
      |                                    ~~~~~^~~~~
a.cc:98:21: error: cannot bind rvalue '(int)(& count)' to 'int&'
   98 |           dfs(i,j,0,&count,S);
      |                     ^~~~~~