Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

type Node struct {
	ID    int
	X, Y  int
	Color Color
	Nexts []*Node
}

func main() {
	for sc.Scan() {
		if sc.Text() == "0 0" {
			break
		}
		chunks := strings.Split(sc.Text(), " ")
		w, _ := strconv.Atoi(chunks[0])
		h, _ := strconv.Atoi(chunks[1])

		// Initialize map
		m := map[int]map[int]*Node{}
		for x := 1; x <= w; x++ {
			m[x] = map[int]*Node{}
			for y := 1; y <= h; y++ {
				m[x][y] = &Node{ID: x + y*w, X: x, Y: y}
			}
		}
		for x := 1; x <= w; x++ {
			for y := 1; y <= h; y++ {
				nexts := []*Node{}
				if n, ok := m[x-1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y-1]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x+1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y+1]; ok {
					nexts = append(nexts, n)
				}
				m[x][y].Nexts = nexts
			}
		}

		// Set colors
		queue := []*Node{}
		touchedIDs := map[int]struct{}{}
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					n := m[i+1][y]
					n.Color = ColorBlack
					if c == '@' {
						queue = append(queue, n)
						touchedIDs[n.ID] = struct{}{}
					}
				case '#':
					m[i+1][y].Color = ColorRed
				}
			}
		}

		// Search
		visitedIDs := map[int]struct{}{}
		for {
			if len(queue) == 0 {
				break
			}
			n := queue[0]
			queue = queue[1:]
			visitedIDs[n.ID] = struct{}{}
			for _, next := range n.Nexts {
				if next.Color == ColorRed {
					continue
				}
				if _, ok := touchedIDs[next.ID]; !ok {
					queue = append(queue, next)
					touchedIDs[n.ID] = struct{}{}
				}
			}
		}
		fmt.Println(len(visitedIDs))
	}
}

C++: #include<iostream>
#include<stdio.h>
#include<math.h>
#include<algorithm>
#include<queue>
#include<deque>
#include<stack>
#include<string>
#include<string.h>
#include<vector>
#include<set>
#include<map>
#include<bitset>
#include<stdlib.h>
#include<cassert>
#include<time.h>
#include<bitset>

int main(){
  while(1){
    int h,w;
    cin>>w>>h;
    if(h==0&&w==0) break;
    char a[h][w];
    queue<pair<int,int>> q;
    bool used[h][w];
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
        cin>>a[i][j];
        used[i][j]=0;
        if(a[i][j]=='@') q.push(make_pair(i,j));
      }
    }
    int ans=0;
    while(!q.empty()){
      pair<int,int> p=q.front();
      q.pop();
      int x=p.first, y=p.second;
      if(used[x][y]) continue;
      used[x][y]=1;
      ans++;
      if(x>0){
        if(!used[x-1][y]&&a[x-1][y]=='.'){
          q.push(make_pair(x-1,y));
        }
      }
      if(x<h-1){
        if(!used[x+1][y]&&a[x+1][y]=='.'){
          q.push(make_pair(x+1,y));
        }
      }
      if(y>0){
        if(!used[x][y-1]&&a[x][y-1]=='.'){
          q.push(make_pair(x,y-1));
        }
      }
      if(y<w-1){
        if(!used[x][y+1]&&a[x][y+1]=='.'){
          q.push(make_pair(x,y+1));
        }
      }
    }
    cout<<ans<<endl;
  }
  return 0;
}

C++: #include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

#define rep(i,a) for(int i = 0 ; i < a ; i ++)
#define loop(i,a,b) for(int i = a ; i < b ; i ++)

int ans;
char maze[27][27];
void dfs(int x,int y){
	int dx[4] = {1,-1, 0, 0};
	int dy[4] = {0, 0, 1, -1};
	maze[x][y]='#';
	rep(i,4){
		int nx = dx[i] + x;
		int ny = dy[i] + y;
		if(maze[nx][ny]=='.'){
			ans++;
			dfs(nx,ny);
		}
	}
}

int main(void){
	int w,h;
	while(cin>>w>>h){
		rep(i,25)rep(j,25)
			maze[i][j]='#';
		ans=0;
		if(w==0&&h==0)return 0;
		rep(i,h)rep(j,w)
			cin>>maze[i][j];
		rep(i,h)rep(j,w){
			if(maze[i][j]=='@'){
				dfs(i,j);
				cout<<ans+1<<endl;
			}
		}
	}
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <cmath>
#include <cstring>
#include <numeric>
#include <algorithm>
#include <functional>
#include <array>
#include <map>
#include <queue>
#include <limits.h>
#include <set>
#include <stack>

using namespace std;

#define Getsign(n) ((n > 0) - (n < 0))

typedef vector<int> Ivec;
typedef pair<int, int> pii;
const pii eight_Dir[4] = { //?????????
	{ 0 ,1 },
	{ -1 ,0 },{ 1 ,0 },
	{ 0 ,-1 }
};
//pii operator+ (pii a, pii b){return { a.first + b.first, a.second + b.second };}
//pii operator- (pii a, pii b) { return { a.first - b.first, a.second - b.second }; }
//pii operator* (pii a, pii b) { return{ a.first * b.first, a.second * b.second }; }
//pii operator/ (pii a, pii b) { return{ a.first / b.first, a.second / b.second }; }
//pii operator% (pii a, pii b) { return{ a.first % b.first, a.second % b.second }; }

int main() {
	int w, h;

	while (scanf("%d %d", &w, &h)) {
		if (w == 0)
			return 0;

		array<array<bool, 22>, 22> map = {}, al = {};
		queue<pii> que;
		for (int i = 1; h >= i; i++) {
			scanf("%*c");
			for (int j = 1; w >= j; j++) {
				char a;
				scanf("%c", &a);

				if (a == '@') {
					que.push({j,i});
					al[j][i] = true;
					map[j][i] = true;
				}
				else if (a == '.') {
					map[j][i] = true;
				}
			}
		}
		int cou = 0;
		while (!que.empty()) {
			pii pos = que.front();
			que.pop();
			cou++;
			for (int i = 0; 4 > i; i++) {
				if (!al[pos.first + eight_Dir[i].first][pos.second + eight_Dir[i].second] && map[pos.first + eight_Dir[i].first][pos.second + eight_Dir[i].second]) {
					al[pos.first + eight_Dir[i].first][pos.second + eight_Dir[i].second] = true;
					que.push({ pos.first + eight_Dir[i].first,pos.second + eight_Dir[i].second});
				}
			}

		}
		printf("%d\n", cou);
	}

	return 0;
}

C++: #include <iostream>

using namespace std;

#define MAX_H 22
#define MAX_W 22

int W, H, X, Y, A;

char tile[MAX_H][MAX_W];

int dx[4] = { 0, 1, 0, -1 };
int dy[4] = { 1, 0, -1, 0 };

void dfs(int x, int y)
{
	tile[y][x] = '@';

	for (int i = 0; i < 4; i++)
	{
		if (0 <= x + dx[i] <= W - 1 && 0 <= y + dy[i] <= H - 1)
		{
			if (tile[y + dy[i]][x + dx[i]] == '.')
			{
				dfs(x + dx[i], y + dy[i]);
			}
		}
	}
}

int main()
{
	while (true)
	{
		cin >> W >> H;

		if (W == 0 && H == 0) { break; }

		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				cin >> tile[i][j];

				if (tile[i][j] == '@')
				{
					X = j;
					Y = i;
				}
			}
		}

		dfs(X, Y);

		A = 0;

		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				if (tile[i][j] == '@')
				{
					A++;
				}
			}
		}

		cout << A << endl;
	}

	return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <queue>
using namespace std;

typedef struct{
	int x;
	int y;
}point;

int main(){
	while(1){
		int w, h;
		char field[30][30];
		int visit[30][30]={0};
		int ans=0;
		int dx[4]={1,0,-1,0}, dy[4]={0,1,0,-1};
		point start;
		
		scanf(" %d %d", &w, &h);
		if(w==0) break;	
		for(int i=0; i<h; ++i){
			for(int j=0; j<w; ++j){
				scanf(" %c", &field[j][i]);		
				if(field[j][i] == '@'){
					start.x=j;
					start.y=i;	
				}
			}	
		}
		
		queue<point> que;
		visit[start.x][start.y]=1;
		que.push(start);
		++ans;
		
		while(!que.empty()){
			point now=que.front();
			que.pop();
			
			for(int i=0; i<4; ++i){
				int nx=now.x+dx[i], ny=now.y+dy[i];
				
				if(nx>=0 && nx<w && ny>=0 && ny<h && field[nx][ny]=='.' && visit[nx][ny]==0){
					visit[nx][ny]=1;
					++ans;
					
					point pp;
					pp.x=nx; pp.y=ny;
					que.push(	pp);
				}
			}
		}
		
		printf("%d\n", ans);
	}
	
	return 0;
}