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 <queue>
#include <string>

using namespace std;

int main(){
  int w,h,count;
  queue<int> que;
  int xx[4]={1,0,-1,0};
  int yy[4]={0,1,0,-1};
  int pos;
  string tmap;
  while(true){
    cin >> w >> h;
    if(!w&&!h)
      break;
    int mmap[w+2][h+2];
    for(int x=0;x<w+2;x++){
      mmap[x][0] = -1;
      mmap[x][h+1] = -1;
    }
    for(int y=0;y<h+2;y++){
      mmap[0][y] = -1;
      mmap[w+1][y] = -1;
    }
        
    for(int y=0;y<h;y++){
      cin >> tmap;
      for(int x=0;x<w;x++){
	switch(tmap[x]){
	case '.':
	  mmap[x+1][y+1] = 0;
	  break;
	case '#':
	  mmap[x+1][y+1] = -1;	  
	  break;
	case '@':
	  mmap[x+1][y+1] = 1;
	  que.push(y*w+x);
	  break;
	}
      }
    }
    count = 1;
    while(!que.empty()){
      pos = que.front();
      for(int dir=0;dir<4;dir++){
	if (mmap[pos%w+xx[dir]+1][pos/w+yy[dir]+1]==0){
	  que.push(pos+yy[dir]*w+xx[dir]);
	  mmap[pos%w+xx[dir]+1][pos/w+yy[dir]+1] = 1;
	  count++;
	}
      }
      que.pop();
    }
    cout << count << endl;
  }
  return 0;
}

C++: #include "bits/stdc++.h"
using namespace std;

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

int main() {
	int W, H;
	while (cin >> W >> H) {
		if (W == 0 && H == 0) return 0;
		vector<string> S(H);
		for (int i = 0; i < H; i++) cin >> S[i];
		int SX, SY;
		for (int i = 0; i < H; i++) {
			for (int j = 0; j < W; j++) {
				if (S[i][j] == '@') SX = i, SY = j;
			}
		}
		queue<pair<int, int> > Q;
		Q.push({ SX, SY });
		while (!Q.empty()) {
			pair<int, int> P = Q.front();
			Q.pop();
			for (int i = 0; i < 4; i++) {
				int NX = P.first + dx[i], NY = P.second + dy[i];
				if (0 <= NX && NX < H && 0 <= NY && NY < W) {
					if (S[NX][NY] == '.') {
						S[NX][NY] = '@';
						Q.push({ NX, NY });
					}
				}
			}
		}
		int ANS = 0;
		for (int i = 0; i < H; i++) {
			for (int j = 0; j < W; j++) {
				if (S[i][j] == '@') ANS++;
			}
		}
		cout << ANS << endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;

#define REP(i, n) for(int i = 0;i < (n);i++)
#define FOR(i, a, b) for(int i = (a);i < (b);i++)
#define all(v) v.begin(),v.end()

using int64 = long long;

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

int main(void){
	int H, W;
	while(cin >> H >> W && H && W){
		vector<string> f(H+2, string(W+2, '#'));
		int sy, sx;
		REP(i, H){
			cin >> f[i+1];
			REP(j, W+2)
				if(f[i+1][j] == '@'){
					sy = i+1;
					sx = j;
				}
			f[i+1][0] = f[i+1][W+1] = '#';
		}
		bool visited[30][30] = {};
		int cnt = 0;
		
		auto dfs = [&](int y, int x){
			visited[y][x] = 1;
			cnt++;
			REP(i, 4){
				int yy = y+dy[i], xx = x+dx[i];
				if(f[yy][xx] != '#' && !visited[yy][xx]){
					dfs(yy, xx);
				}
			}
		};
		dfs(sy, sx);
		cout << cnt << endl;
	}
}'

C++: #include <algorithm>
#include <cstring>
#include <iostream>
#include <vector>
#include <cmath>
#include <math.h>
#include <utility>
using namespace std;

typedef long long int ll; 
typedef pair<int,int> Pii;

int W,H,tmpAns;
vector <vector <int> > cels;
vector<int> ans;
vector<Pii> candidate;//未探索候補

// タイルの種類　黒＝1，赤＝2，探索済み＝3

bool isCan(int h,int w){
    // 範囲チェック
    if(h<0||h>H-1)return false;
    if(w<0||w>W-1)return false;

    // チェック
    if(cels[h][w]==1)return true;
    return false;
}

void searchCel(int h,int w){
Pii tmpPair;

tmpAns++;
cels[h][w]=3;
if(isCan(h,w+1)==true){
    tmpPair.first = h;tmpPair.second = w+1;
    candidate.push_back(tmpPair);
}
if(isCan(h,w-1)==true){
    tmpPair.first = h;tmpPair.second = w-1;
    candidate.push_back(tmpPair);
}
if(isCan(h+1,w)==true){
    tmpPair.first = h+1;tmpPair.second = w;
    candidate.push_back(tmpPair);
}
if(isCan(h-1,w)==true){
    tmpPair.first = h-1;tmpPair.second = w;
    candidate.push_back(tmpPair);
}

while(1){
    if(candidate.empty())return;
    pair<int,int> tmpPair=candidate.back();
    candidate.pop_back();
    if(cels[tmpPair.first][tmpPair.second]==1){
        searchCel(tmpPair.first,tmpPair.second);
        break;
    }
}

return;
}

int main(){
    char tmp;
    int now_w,now_h;
    while(1){
        cin>>W>>H;
        if(W==0&&H==0)break;
        cels.clear();
        cels.assign(H,vector<int>(W,0));
        tmpAns=0; 
        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                cin>>tmp;
                switch (tmp){
                case '.':
                    cels[i][j]=1;
                    break;
                case '#':
                    cels[i][j]=2;
                    break;
                case '@':
                    cels[i][j]=3;
                    now_h=i;now_w=j;
                    break;
                default:
                    return -1;
                }
            }
        }
        searchCel(now_h,now_w);
        ans.push_back(tmpAns);
        
    }

    for(int i=0;i<ans.size();i++){
        cout<<ans[i]<<"\n";
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <utility>
#include <stack>

using namespace std;

int mapToNum(char c)
{
	switch (c) {
	case '#':
			return -1;
	case '.':
			return 0;
	case '@':
			return 1;
	default:
			return -2;
	}
}

int main()
{
	while (1) {
		int w, h;
		
		cin >> w >> h;
		
		if (w == 0 && h == 0) break;
		
		stack< pair<int, int> > stk;
		vector< vector<int> > tiles(h + 2, vector<int>(w + 2));
		char c;
		
		for (int i = 0; i < w + 2; ++i) tiles.at(0).at(i) = tiles.at(h + 1).at(i) = -1;
		
		for (int i = 1; i < h + 1; ++i) {
			tiles.at(i).at(0) = tiles.at(i).at(w + 1) = -1;
			for (int j = 1; j < w + 1; ++j) {
				cin >> c;
				tiles.at(i).at(j) = mapToNum(c);
				if (c == '@') stk.push(make_pair(i, j));
			}
		}
		
		int num = 0;
		
		while (!stk.empty()) {
			pair<int, int> p = stk.top();
			stk.pop();
			int x = 1, y = 0, t;
			
			for (int i = 0; i < 4; ++i) {
				if (tiles.at(p.first + x).at(p.second + y) != -1) {
					tiles.at(p.first + x).at(p.second + y) = -1;
					stk.push(make_pair(p.first + x, p.second + y));
					++num;
				}
				
				t = -y;
				y = x;
				x = t;
			}
		}
		
		cout << num << endl;
	}
	
	return 0;
}