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 <sstream>
#include <string>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <algorithm>
#include <cstdio>
#include <cmath>

using namespace std;

#define all(c) ((c).begin()),((c).end())
#define debug(c) cerr<<"> "<<#c<<" = "<<(c)<<endl;
#define iter(c) __typeof((c).begin())
#define tr(i,c) for(iter(c) i=(c).begin();i!=(c).end();i++)
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define REP(i,a,b) for(int i=(int)(a);i<=(int)(b);i++)

#define mp make_pair
#define fst first
#define snd second
#define pb push_back

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;

int go(int x, int y, char m[21][21], int w, int h) {
	int dx[] = {1, -1, 0, 0};
	int dy[] = {0, 0, 1, -1};
	m[y][x] = '#';
	int res = 1;
	rep (i, 4) {
		int tx = x + dx[i];
		int ty = y + dy[i];
		if(tx < 0 || tx >= w || ty < 0 || ty >= h) continue;
		if(m[ty][tx] == '.') res += go(tx, ty, m, w, h);
	}
	return res;
}

int main(){
	for (int w, h; cin >> w >> h, (w | h) != 0; ) {
		char m[21][21];
		int sx, sy;
		rep (i, h) rep (j, w) {
			cin >> m[i][j];
			if (m[i][j] == '@') sx = j, sy = i;
		}
		cout << go(sx, sy, m, w, h) << endl;
	}
	
	return 0;
}

C++: #include<bits/stdc++.h>
#include<vector>
using namespace std;
//上下右左を差すための配列
int keep_nx[4] = {1 , 0 , -1 , 0};
int keep_ny[4] = {0 , 1 , 0 , -1};
vector< vector<bool> > can_Go (20 , vector<bool> (20));   //フィールドで行ける場所

//深さ優先探索
void dfs(int x, int y, int height, int width, int *cnt){
  can_Go[y][x] = false;   *cnt += 1;
  for(int i = 0 ; i < 4 ; i++){
      int nx = x + keep_nx[i], ny = y + keep_ny[i];
      if(0 <= nx && nx < width && 0 <= ny && ny < height && can_Go[ny][nx] == true)
        dfs(nx, ny, height, width, cnt);
  }
}


int main(void){

while(true){
  int width, height, start_x, start_y;    cin >> width >> height;
  if(width == 0 && height == 0)   break;

  for(int i = 0 ; i < height ; i++){
    for(int j = 0 ; j < width ; j++){
      char input;   cin >> input;
      if(input == '.')  can_Go[i][j] = true;
      if(input == '#')  can_Go[i][j] = false;
      if(input == '@'){
        start_x = j;  start_y = i;
      }
    }
  }

  int ans = 0;
  dfs(start_x, start_y, height, width, &ans);

  cout << ans << endl;
}

  return 0;
}

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

using ll = long long;
#define debug(x) cerr << #x << ": " << x << endl;
#define line() cerr << "---------------" << endl;

char board[30][30];
bool visited[30][30];

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

void dfs(int x, int y) {
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= H || ny < 0 || ny >= W) continue;
      if (board[nx][ny] == '#') continue;
      if (visited[nx][ny]) continue;
      visited[nx][ny] = true;
      dfs(nx, ny);
   }
}

int main() {
   while (cin >> W >> H, W || H) {
      memset(visited, false, sizeof(visited));

      int sx = 0, sy = 0; // 人の座標
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            cin >> board[i][j];
            if (board[i][j] == '@') {
               sx = i, sy = j;
            }
         }
      }

      visited[sx][sy] = true;
      dfs(sx, sy);

      int ans = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (visited[i][j]) ans++;
         }
      }
      cout << ans << endl;

   }

   return 0;
}