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++: #if 0
08-162015?????????????????°???????????????????????????
pair???
#endif

#include <iostream>
#include <queue>
#include <string.h>
#include <utility>
using namespace std;

int w,h;
int tot=1; //?????£????????????(?????????????????????)
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1}; //??????????§??????¨
char P[100][100]; //??§?¨?????????????????????¨???

bool valid(int x,int y){ //??????????????????
	return (x>=0)&&(x<w)&&(y>=0)&&(y<h)&&(P[x][y]!='#');
}

void bfs(int x,int y){ //???????????¢?´¢
	queue<pair<int,int> > q;
	pair<int,int> p;
	p = make_pair(x,y); //pair??????????????¨??????????????????
	q.push(p);
	while(!q.empty()){
		pair<int,int> cur=q.front(); //?????¨??°?????????????????????
		q.pop();
		for(int i=0;i<4;++i){
			if(valid(cur.first+dx[i],cur.second+dy[i])){
				q.push(make_pair(cur.first+dx[i],cur.second+dy[i]));
				tot++;
				P[cur.first+dx[i]][cur.second+dy[i]]='#'; //???????????£?????´???????°???????
			}
		}
		P[cur.first][cur.second]='#';
	}
}

int main(){
	int x1,y1;
	while((cin>>w>>h)&&(w!=0)){
		for(int j=0;j<h;++j){
			for(int i=0;i<w;++i){
				cin>>P[i][j];
				if(P[i][j]=='@')
					x1=i; y1=j; 
			}
		}
		bfs(x1,y1); //??????????????°????????£??\????????¢?´¢????§?
	}
	cout << tot <<endl;
}

C++: #include <iostream>

using namespace std;

char tile[20][20];
int canVisit[20][20];

void bfs(int x, int y, int w, int h)
{  
  if (tile[y][x] == '#') return;
  if (canVisit[y][x] == 1) return;
  
  if (canVisit[y][x] == 0) {
    canVisit[y][x] = 1;
  }
  
  int dx[] = { -1,  0,  0,  1 };
  int dy[] = {  0,  1, -1,  0 };

  for (int i = 0; i < 4; i++) {
    for (int j = 0; j < 4; j++) {
      if (x + dx[i] >= 0 && x + dx[i] < w
          && y + dy[i] >= 0 && y + dy[i] < h) {
        bfs(x + dx[i], y + dy[i], w, h);

      }
    }
  }
}

int main()
{
  int w, h;
  int init_x, init_y;
  
  while (1) {
    cin >> w >> h;
    if (w == 0 && h == 0) break;

    for (int i = 0; i < 20; i++) {
      for (int j = 0; j < 20; j++) {
        tile[i][j] = '#';
        canVisit[i][j] = 0;
      }
    }
    
    for (int i = 0; i < h; i++) {
      for (int j = 0; j < w; j++) {
        cin >> tile[i][j];
        if (tile[i][j] == '@') {
          init_x = j;
          init_y = i;
        }
      }
    }

    bfs(init_x, init_y, w, h);
    
    int sum_of_tile = 0;
    for (int i = 0; i < h; i++) {
      for (int j = 0; j < w; j++) {
        sum_of_tile += canVisit[i][j];
      }
    }
    cout << sum_of_tile << endl;
  }
}

C++: #include <iostream>

using namespace std;

int W, H;
int x, y;
char tile[20][20];
int checked[20][20];
int dfs(int x, int y){
    checked[x][y] = 1;
    bool dead = false;
    if((x==0 || tile[x-1][y] == '#' || checked[x-1][y] == 1) &&
    (x==W-1 || tile[x+1][y] == '#' || checked[x+1][y] == 1) &&
    (y==0 || tile[x][y-1] == '#' || checked[x][y-1] == 1) &&
    (y==H-1 || tile[x][y+1] == '#' || checked[x][y+1] == 1)) dead = true;
    if(dead){
        return 1;
    }
    int tmp=0;
    if(x>0 && tile[x-1][y] == '.' && checked[x-1][y] == 0) tmp += dfs(x-1, y);
    if(x<W-1 && tile[x+1][y] == '.' && checked[x+1][y] == 0) tmp += dfs(x+1, y);
    if(y>0 && tile[x][y-1] == '.' && checked[x][y-1] == 0) tmp += dfs(x, y-1);
    if(y<H-1&&tile[x][y+1] == '.' && checked[x][y+1] == 0) tmp += dfs(x, y+1);
    return 1+tmp;
}
int main(){
    char tmp;
    while(cin >> W >> H && W && H){
        for(int j=0; j<H; ++j){
            for(int i=0; i<W; ++i){
                checked[i][j] = 0;
                cin >> tmp;
                switch(tmp){
                    case '.':
                        tile[i][j] = '.';
                        break;
                    case '#':
                        tile[i][j] = '#';
                        break;
                    case '@':
                        tile[i][j] = '.';
                        x = i;
                        y = j;
                        break;
                }
            }
        }
        cout << dfs(x, y) << endl;
    }
    return 0;
}