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
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <queue>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const int DX[] = {0, 1, 0, -1};
const int DY[] = {-1, 0, 1, 0};


//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int main(int argc, char const *argv[])
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	for(int h, w; cin >> w >> h, w | h;){
		VVI table(h, VI(w));
		PII goal;
		REP(i, h){
			string line;
			cin >> line;
			REP(j, w){
				char letter = line[j];
				int value = 0;
				if(letter == '@'){
					goal = MP(i, j);
					value = -1;
				}else if(letter == '#'){
					value = -1;
				}
				table[i][j] = value;
			}
		}
		queue<PII> q;
		q.push(goal);
		int total = 1;
		while(!q.empty()){
			PII p = q.front();
			q.pop();
			int x = p.second, y = p.first;
			int cv = table[y][x];
			REP(i, 4){
				int dx = x + DX[i];
				int dy = y + DY[i];
				if(0 <= dx && dx < w &&
					0 <= dy && dy < h &&
					table[dy][dx] == 0){
					total++;
					table[dy][dx] = 1;
					q.push(MP(dy, dx));
				}
			}
		}

		cout << total << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <string>
using namespace std;
vector<int> x;
vector<int> y;
vector<int> nextx;
vector<int> nexty;
int w,h;
int count;
bool check(int a,int b){
  for (int i=0;i<nextx.size();i++){
    if (nextx[i]==a&&nexty[i]==b)
      return false;
  }
  return true;
}
int search(int **field){
  x.clear();
  y.clear();
  for (int i=0;i<nextx.size();i++){
    x.push_back(nextx[i]);
    y.push_back(nexty[i]);
  }
  nextx.clear();
  nexty.clear();
  for (int i=0;i<x.size();i++){
    field[x[i]][y[i]]=0;
    count++;
  }
  for (int i=0;i<x.size();i++){
    if (field[x[i]-1][y[i]]==1&&x[i]-1>=0&&check(x[i]-1,y[i])){
      nextx.push_back(x[i]-1);
      nexty.push_back(y[i]);
    }
    if (x[i]+1<h&&field[x[i]+1][y[i]]==1&&check(x[i]+1,y[i])){
      nextx.push_back(x[i]+1);
      nexty.push_back(y[i]);
    }
    if (field[x[i]][y[i]-1]==1&&y[i]-1>=0&&check(x[i],y[i]-1)){
      nextx.push_back(x[i]);
      nexty.push_back(y[i]-1);
    }
    if (field[x[i]][y[i]+1]==1&&y[i]+1<w&&check(x[i],y[i]+1)){
      nextx.push_back(x[i]);
      nexty.push_back(y[i]+1);
    }
  }
  if (nextx.empty()){
    return -1;
  }
  int *f[h];
  for (int i=0;i<h;i++)
    f[i]=field[i];
  return search(f);
}
int main(){
  string temp;
  while (cin>>w>>h,w|h!=0){
    count=0;
    int field[h][w];
    for (int i=0;i<h;i++){
      cin>>temp;
      for (int j=0;j<w;j++){
	if (temp[j]=='.')
	  field[i][j]=1;
	if (temp[j]=='#')
	  field[i][j]=0;
	if (temp[j]=='@'){
	  field[i][j]=1;
	  nextx.push_back(i);
	  nexty.push_back(j);
	}
      }
    }
    int *f[h];
    for (int i=0;i<h;i++)
      f[i]=field[i];
    search(f);
    cout<<count<<endl;
  }
}

C++: #include<cstdio>
#include<cstdlib>
#include<iostream>
#include<string>
#include<map>
#include<vector>
#include<algorithm>
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)

using namespace std;
int ans = 0;

void bfs(char **tile, int w, int h){
  ans++;
  tile[w][h] = '#';

  if(tile[w][h-1] == '.')
  bfs(tile, w, h-1);

  if(tile[w][h+1] == '.')
  bfs(tile, w, h+1);

  if(tile[w-1][h] == '.')
  bfs(tile, w-1, h);

  if(tile[w+1][h] == '.')
  bfs(tile, w+1, h);

  return;
}


int main(){
  int W,H,index_w,index_h;

  while(true){
    cin >> W >> H;
    if(W == 0 && H == 0) break;

    char **tile;
    tile = new char* [H+2];
    REP(i,H+2) tile[i] = new char[W+2];

    REP(i,H+2){
      REP(j,W+2){
        if(i == 0 || i == H+1 || j == 0 || j == W+1) tile[i][j] = '#'; //sentry?
        else scanf(" %c",&tile[i][j]);

        if(tile[i][j] == '@') {
          index_h = j;
          index_w = i;
        }

      }
    }
    count = 0;
    bfs(tile,index_w,index_h);
    
    REP(i,H) delete[] tile[i];
    delete[] tile;
    
    cout << ans << endl;
  }
  return 0;
}