Go: package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
)

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

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

var strByColor = map[Color]string{
	ColorBlack: "black",
	ColorRed:   "red",
}

func (c Color) String() string {
	return strByColor[c]
}

type Node struct {
	ID      int
	X, Y    int
	Color   Color
	NextIDs map[int]struct{}
}

func (n *Node) String() string {
	d, _ := json.Marshal(n)
	return string(d)
}

type Map struct {
	Width, Height int
	Nodes         map[int]*Node
}

func (m *Map) GetByID(id int) *Node {
	return m.Nodes[id]
}

func (m *Map) Get(x, y int) *Node {
	return m.Nodes[m.getID(x, y)]
}

func (m *Map) SetColor(x, y int, c Color) {
	m.Nodes[m.getID(x, y)].Color = c
}

func (m *Map) getID(x, y int) int {
	return x + y*m.Width
}

func (m *Map) init() {
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			id := m.getID(x, y)
			m.Nodes[id] = &Node{ID: id, X: x, Y: y}
		}
	}
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			nextIDs := map[int]struct{}{}
			if x > 1 {
				nextIDs[m.getID(x-1, y)] = struct{}{}
			}
			if x < m.Width {
				nextIDs[m.getID(x+1, y)] = struct{}{}
			}
			if y > 1 {
				nextIDs[m.getID(x, y-1)] = struct{}{}
			}
			if y < m.Height {
				nextIDs[m.getID(x, y+1)] = struct{}{}
			}
			m.Get(x, y).NextIDs = nextIDs
		}
	}
}

func NewMap(w, h int) *Map {
	m := &Map{Width: w, Height: h, Nodes: map[int]*Node{}}
	m.init()
	return m
}

type Searcher struct {
	Map        *Map
	queue      []*Node
	touchedIDs map[int]struct{}
	visitedIDs map[int]struct{}
}

func NewSearcher(m *Map) *Searcher {
	return &Searcher{
		Map:        m,
		queue:      []*Node{},
		touchedIDs: map[int]struct{}{},
		visitedIDs: map[int]struct{}{},
	}
}

func (s *Searcher) Search(start *Node) {
	s.enqueue(start)

	for {
		if len(s.queue) == 0 {
			break
		}
		for nextID := range s.dequeue().NextIDs {
			n := s.Map.GetByID(nextID)
			if n.Color == ColorRed || s.hasTouched(n) {
				continue
			}
			s.enqueue(n)
		}
	}
}

func (s *Searcher) VisitedNodeCount() int {
	return len(s.visitedIDs)
}

func (s *Searcher) hasTouched(n *Node) bool {
	_, ok := s.touchedIDs[n.ID]
	return ok
}

func (s *Searcher) enqueue(n *Node) {
	s.queue = append(s.queue, n)
	s.touchedIDs[n.ID] = struct{}{}
}

func (s *Searcher) dequeue() *Node {
	n := s.queue[0]
	s.queue = s.queue[1:]
	s.visitedIDs[n.ID] = struct{}{}
	return n
}

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])

		m := NewMap(w, h)

		// Set colors
		var start *Node
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					m.SetColor(i+1, y, ColorBlack)
					if c == '@' {
						start = m.Get(i+1, y)
					}
				case '#':
					m.SetColor(i+1, y, ColorRed)
				}
			}
		}

		// Search
		s := NewSearcher(m)
		s.Search(start)
		fmt.Println(s.VisitedNodeCount())
	}
}

C++: #include<iostream>
#include<string>
#include<queue>
#include<vector>
using namespace std;
#define frp(i,n) for((i)=0;(i)<(n);i++)
int vx[4]={0,1,0,-1};
int vy[4]={1,0,-1,0};
int H,W;
vector<int> red1;
vector<int> red2;
int check(int n1,int n2){
  for(int i=0;i<red1.size();i++){
    if(n1==red1[i]&&n2==red2[i]||n1<0||n2<0||n1>H||n2>W)
      return 0;
  }
  return 1;
}
int main(){
  int i,j;
  string str2;
  vector<string> str;
  while(cin>>W>>H,W+H!=0){
    int num=1;
    frp(i,H){
      cin>>str2;
      str.push_back(str2);
    }
    frp(i,H){
      frp(j,W){
	if(str[i][j]=='#'){
	  red1.push_back(i);
	  red2.push_back(j);
	}
      }
    }
    queue<pair<int,int> > qii;
    frp(i,H){
      frp(j,W){
	if(str[i][j]=='@')
	  qii.push(make_pair(i,j));
      }
    }
    while(!qii.empty()){
      int now1=qii.front().first;
      int now2=qii.front().second;
      qii.pop();//?????????????´??????´?£?
      for(i=0;i<4;i++){
	if(check(now1+vx[i],now2+vy[i])==1){
	  red1.push_back(now1);
	  red2.push_back(now2);
	  qii.push(make_pair(now1+vx[i],now2+vy[i]));
	  num++;
	}
      }
    }
    cout<<num<<endl;
  }
  return 0;
}

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

#define ll long long
#define vvi vector<vector<int>>
#define vec vector
#define pq priority_queue
#define all(v) (v).begin(), (v).end()
#define rep(i, n) for (int (i) = (0); (i) < (n); ++(i))
#define repp(i, m, n) for (int (i) = (m); (i) < (n); ++(i))
#define debug(x) cerr << #x << ": " << x << endl;
#define debug2(x, y) cerr<<"("<<#x<<", "<<#y<<") = "<<"("<<x<<", "<<y<<")"<<endl;
#define debug3(x, y, z) cerr<<"("<<#x<<", "<<#y<<", "<<#z<<") = "<<"("<<x<<", "<<y<<", "<<z<<")"<<endl;
#define debugB(x, y) cerr<<#x<<": "<<bitset<y>(x) << endl;
#define line() cerr << "---------------" << endl;

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

template<typename T>
void printA(vector<T> &printArray, char between = ' ') {
   int paSize = printArray.size();
   for (int i = 0; i < paSize; i++) { cerr << printArray[i] << between;}
   if (between != '\n') {cerr << endl;}
}

// ------------------------------------------------------------------------------------------

int W, H;
char B[30][30];

int dfs(int y, int x) {
   int ret = 1;
   B[y][x] = '#';
   rep (i, 4) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= W || ny < 0 || ny >= H || B[ny][nx] == '#') continue;
      ret += dfs(ny, nx);
   }
   return ret;
}

void solve() {
   rep (i, H) {
      cin >> B[i];
   }
   rep (i, H) {
      rep (j, W) {
         if (B[i][j] == '@') {
            cout << dfs(i, j) << endl;
            return;
         }
      }
   }
}

int main() {
   while (1) {
      cin >> W >> H;
      if (H*W == 0) break;
      solve();
   }

   return 0;
}

C++: #include <stdio.h>
#include <stack>
#include <math.h>
#include <queue>

using namespace std;

int W,H;
bool visited[20][20];
char table[20][21];

struct Tile{
	int row,col;
};

bool rangeCheck(int row,int col){
	if(row >= 0 && row <= H-1 && col >=0 && col <= W-1)return true;
	else{
		return false;
	}
}

void func(int row,int col){
	queue<Tile> Q;
	Tile tile;
	tile.row = row;
	tile.col = col;

	Q.push(tile);

	Tile tmp;
	while(!Q.empty()){
		tmp = Q.front();
		Q.pop();

		if(rangeCheck(tmp.row-1,tmp.col) == true && visited[tmp.row-1][tmp.col] == false && table[tmp.row-1][tmp.col] == '.'){
			visited[tmp.row-1][tmp.col] = true;

			Tile newTmp;
			newTmp.row = tmp.row-1;
			newTmp.col = tmp.col;
			Q.push(newTmp);
		}
		if(rangeCheck(tmp.row+1,tmp.col) == true && visited[tmp.row+1][tmp.col] == false && table[tmp.row+1][tmp.col] == '.'){
			visited[tmp.row+1][tmp.col] = true;
			Tile newTmp2;
			newTmp2.row = tmp.row+1;
			newTmp2.col = tmp.col;
			Q.push(newTmp2);
		}
		if(rangeCheck(tmp.row,tmp.col-1) == true && visited[tmp.row][tmp.col-1] == false && table[tmp.row][tmp.col-1] == '.'){
			visited[tmp.row][tmp.col-1] = true;
			Tile newTmp3;
			newTmp3.row = tmp.row;
			newTmp3.col = tmp.col-1;
			Q.push(newTmp3);
		}
		if(rangeCheck(tmp.row,tmp.col+1) == true && visited[tmp.row][tmp.col+1] == false && table[tmp.row][tmp.col+1] == '.'){
			visited[tmp.row][tmp.col+1] = true;
			Tile newTmp4;
			newTmp4.row = tmp.row;
			newTmp4.col = tmp.col+1;
			Q.push(newTmp4);
		}
	}
}

int main(){

	int x,y,count;

	while(true){
		scanf("%d %d",&W,&H);
		if(W == 0 && H == 0)break;

		for(int i = 0; i < H; i++){
			for(int k = 0; k < W; k++)visited[i][k] = false;
		}

		for(int i = 0; i < H; i++){
			scanf("%s",table[i]);

			for(int k = 0; k < W; k++){
				if(table[i][k] == '@'){
					y = i;
					x = k;
				}
			}
		}

		visited[y][x] = true;
		func(y,x);

		count = 0;

		for(int i = 0; i < H; i++){
			for(int k = 0; k < W; k++){
				if(visited[i][k] == true)count++;
			}
		}

		printf("%d\n",count);

	}


    return 0;
}