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 <deque>
#include <iostream>
#include <string>
#include <vector>

#define rep(i, n) for (int i = 0; i < int(n); i++)

using namespace std;

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

int red_black(point size, point sp, vector<vector<char>> &room) {
  deque<point> queue(0);
  queue.push_front(sp);
  int sum = 0;

  while (queue.size() > 0) {
    auto p = queue.front();
    queue.pop_front();

    auto x = p.x;
    auto y = p.y;
    if (x < 0 || y < 0 || x == size.x || y == size.y) {
      continue;
    }

    auto &tile = room[y][x];
    if (tile == '#') {
      continue;
    }

    sum += 1;
    tile = '#';
    queue.push_front(point{x + 1, y});
    queue.push_front(point{x - 1, y});
    queue.push_front(point{x, y + 1});
    queue.push_front(point{x, y - 1});
  }

  return sum;
}

int main() {
  int w, h, sx, sy;
  char c;
  while (true) {
    cin >> w >> h;
    if (w == 0) {
      break;
    }

    vector<vector<char>> room(h, vector<char>(w));
    rep(y, h) {
      rep(x, w) {
        cin >> c;
        if (c == '@') {
          sx = x;
          sy = y;
        }
        room[y][x] = c;
      }
    }

    auto result = red_black(point{w, h}, point{sx, sy}, room);
    cout << result << endl;
  }
}

C++: #include <iostream>
using namespace std;
char S[21][21];
int count = 0;

void a(int i, int j){
	if(S[i][j] == '.'){
		count++;
		S[i][j] = '#';
		a(i - 1, j);
		a(i + 1, j);
		a(i, j + 1);
		a(i, j - 1);
	}
}


int main(){
	while(true){
		memset(S,'#',sizeof(S));
		count = 0;
		int x, y;
		cin >> x >> y;
		if(x == 0 && y == 0){
			break;
		}
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				cin >> S[i][j];
			}
		}

		int startx, starty;
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				if(S[i][j] == '@'){
					startx = j;
					starty = i;
					S[i][j] = '.';
					break;
				}
			}
		}
		a(starty, startx);
		cout << count << endl;
	}
}

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

typedef pair<int, int> P;
const int W = 20, H = 20;
char board[H][W];

int bfs(int x, int y, int w, int h){
	int dx[] = {0, -1, 1, 0}, dy[] = {-1, 0, 0, 1}, c = 0;
	queue<P> q;
	q.push(make_pair(x, y));
	board[y][x] = '#';
	while(!q.empty()){
		P p = q.front();
		q.pop();
		++c;
		for(int i = 0;i < 4;++i){
			int nx = p.first + dx[i], ny = p.second + dy[i];
			if(0 <= nx && nx < w && 0 <= ny && ny < h && board[ny][nx] == '.'){
				q.push(make_pair(nx, ny));
				board[ny][nx] = '#';
			}
		}
	}
	return c;
}

int main(){
	int w, h, x, y;
	while(cin >> w >> h, w){
		for(int i = 0;i < h;++i){
			for(int j = 0;j < w;++j){
				cin >> board[i][j];
				if(board[i][j] == '@'){
					x = j;
					y = i;
				}
			}
		}
		cout << bfs(x, y, w, h) << endl;
	}
	return 0;
}

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

const int MAX_SIZE = 20;
typedef pair<int, int> P;

int calc(char f[][MAX_SIZE], queue<P> q, int W, int H) {
  set<P> s;
  while (q.size()) {
    P p = q.front();
    q.pop();
    s.insert(p);
    int di[] = {-1, 1, 0, 0};
    int dj[] = {0, 0, -1, 1};
    for (int k = 0; k < 4; ++k) {
      int i = p.first  + di[k];
      int j = p.second + dj[k];
      if (i >= 0 && j >= 0 && i < H && j < W && f[i][j] == '.') {
        pair<set<P>::iterator, bool> r = s.insert(P(i, j));
        if (r.second) q.push(*r.first);
      }
    }
  }
  return s.size();
}
int main() {
  int W, H;
  char f[MAX_SIZE][MAX_SIZE];
  char c[MAX_SIZE + 1];
  while (true) {
    queue<P> q;
    cin >> W >> H;
    if (W == 0 && H == 0) break;
    for (int i = 0; i < H; ++i) {
      cin >> c;
      for (int j = 0; j < W; ++j) {
        f[i][j] = c[j];
        if (f[i][j] == '@') {
          q.push(P(i, j));
        }
      }
    }
    cout << calc(f, q, W, H) << endl;
  }
}