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++: /**********************************************

1.red and black
2.炭田高輝
3.sumita.takaki@gmail.com

last updated: 2013-10-15@20:19

 **********************************************/
#include<iostream>
#include<algorithm>
#include<sstream>
#include<vector>
#include<queue>
#include<deque>
#include<map>
#include<set>
#include<string>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<cctype>
using namespace std;


void rec(int y, int x);
int dy[4] = {-1, 0, 1, 0};
int dx[4] = {0, 1, 0, -1};
bool visit[21][21];
string tiles[21];
int h=0, w=0;



int main(void){
        


        while(cin >> w >> h, h){
                
                
                for(int i=0; i<h; i++){

                        cin >> tiles[i];
                        
                        for(int j=0; j<w; j++)visit[i][j] = false;
                }


                for(int i=0; i<h; i++){

                        for(int j=0; j<w; j++){
                                
                                if(tiles[i][j] == '@')rec(i,j);

                        }
                }
                                
                int ans = 0;

                for(int i=0; i<h; i++){

                        for(int j=0; j<w; j++){
                                if(visit[i][j])ans++;
                        }

                }

                cout << ans << endl;

        }



}

void rec(int y, int x){

        
        if(visit[y][x])return;
        visit[y][x] = true;

        for(int i=0; i<4; i++){
                int ny = y + dy[i], nx = x + dx[i];

                if(ny<0 || nx<0 || ny>=h || nx>=w)continue;
                if(tiles[ny][nx] == '#')continue;
                rec(ny,nx);
        }



}

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

using namespace std;

vector<string> field;
int w,h;

int bfs(int sx,int sy){

	bool passed[21][21];
	for(int i = 0; i < 21; i++)
		fill(passed[i],passed[i]+21,false);

	queue<pair<int,int> > *prv = new queue<pair<int,int> >();
	queue<pair<int,int> > *nxt = new queue<pair<int,int> >();

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

	prv->push(make_pair(sy,sx));
	while(prv->size()){
		while(prv->size()){
			pair<int,int> cp = prv->front();prv->pop();
			if(passed[cp.first][cp.second])
				continue;
			passed[cp.first][cp.second] = true;
			for(int i = 0; i < 4; i++){
				pair<int,int> np = cp;
				np.first += dy[i];
				np.second += dx[i];
				// ツ各ツ陛サツ古シツづ鳴暗堋督ョ
				if(np.first >= 0 && np.second >= 0 && np.first < h && np.second < w){
					if(passed[np.first][np.second] || field[np.first][np.second] == '#')
						continue;
					nxt->push(np);
				}
			}
		}
		swap(prv,nxt);
	}

	delete prv,nxt;

	int cnt = 0;
	for(int i = 0; i < h; i++){
		for(int j = 0; j < w; j++){
			if(passed[i][j])
				cnt++;
		}
	}

	return cnt;
}

int main(){

	while(cin >> w >> h && !(w == 0 && h == 0)){
		int sx,sy;
		for(int i = 0; i < h; i++){
			string str;
			cin >> str;
			field.push_back(str);
			for(int k = 0; k < str.size(); k++){
				if(str[k] == '@'){
					sx = k;
					sy = i;
				}
			}
		}
		cout << bfs(sx,sy) << endl;

		field.clear();
	}

	return 0;
}

C++: #include <iostream>
#include <vector>
#include <queue>
using namespace std;
typedef pair<int,int> P;

int main()
{
  int w, h;
  while (cin >> w >> h && w != 0) {
    vector<string> grid(h);
    P start;
    for (int i = 0; i < h; i++) {
      cin >> grid[i];
      for (int j = 0; j < w; j++) {
        if (grid[i][j] == '@') {
          start = make_pair(i, j);
          grid[i][j] = '#';
        }
      }
    }

    queue<P> q;
    q.push(start);
    int ans = 0;
    while (!q.empty()) {
      const P p = q.front();
      q.pop();

      ans++;
      for (int d = 0; d < 4; d++) {
        static const int dir_h[] = {-1, 1, 0, 0};
        static const int dir_w[] = {0, 0, -1, 1};
        const int next_h = p.first+dir_h[d];
        const int next_w = p.second+dir_w[d];
        if (next_h >= 0 && next_h < h && next_w >= 0 && next_w < w
            && grid[next_h][next_w] == '.') {
          q.push(make_pair(next_h, next_w));
          grid[next_h][next_w] = '#';
        }
      }
    }
    cout << ans << endl;
  }
  return 0;
}