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<algorithm>
#include<vector>
#include<map>
#include<queue>
using namespace std;
#define REP(i, a, n) for(int i=a; i<n; i++)
#define INF 1000000000


int dy[4] = { -1, 0, 1, 0 };
int dx[4] = { 0, 1, 0, -1 };

int main() {
	int W, H;
	while (cin >> W >> H, W || H) {
		int fy, fx;
		char field[21][21];
		bool visit[21][21] = {};
		REP(i, 0, H) REP(j, 0, W) {
			cin >> field[i][j];
			if (field[i][j] == '@') {
				fy = i;
				fx = j;
				visit[i][j] = true;
			}
		}

		queue<pair<int, int>> q;
		pair<int, int> pii;
		q.push(make_pair(fy, fx));
		visit[fy][fx] = true;

		while (!q.empty()) {
			pii = q.front();
			q.pop();

			REP(k, 0, 4) {
				int y = pii.first + dy[k];
				int x = pii.second + dx[k];
				if (y < 0 || x < 0 || y >= H || x >= W) continue;
				if (field[y][x] == '#' || visit[y][x]) continue;

				visit[y][x] = true;
				q.push(make_pair(y, x));
			}
		}

		int ans = 0;
		REP(i, 0, H) {
			REP(j, 0, W) {
				if (visit[i][j]) ans++;
			}
		}
		cout << ans << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>

using namespace std;

int h, w, sy, sx, dy[] = {-1, 0, 1, 0}, dx[] = {0, 1, 0, -1};
char data[20][20];
bool vis[20][20];

bool check(int y, int x){
  if(y<0 || y>=h) return false;
  if(x<0 || x>=w) return false;
  return true;
}

int dfs(int y, int x){
  int res = 1;
  vis[y][x] = true;
  for(int i=0;i<4;i++){
    int ny = y + dy[i];
    int nx = x + dx[i];
    if(!check(ny, nx)) continue;
    if(vis[ny][nx]) continue;
    if(data[ny][nx] == '.') res += dfs(ny, nx);
  }
  return res;
}

main(){
  while(cin >> w >> h && (w|h)){
    fill(vis[0], vis[20], false);
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
        cin >> data[i][j];
        if(data[i][j] == '@'){
          sy = i;
          sx = j;
          data[i][j] = '.';
        }
      }
    }
    cout << dfs(sy, sx) << endl;
  }
}

C++: /*
 * http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp
 */


#include <iostream>
#include <limits>

constexpr size_t MAX_HEIGHT = 20;
const char START_TILE = '@';
const char PASSABLE_TILE = '.';
const char PROHIBITED_TILE = '#';
const char PASSED_TILE = 'P';

bool is_wall(std::string world[MAX_HEIGHT], size_t x, size_t y) {
	char tile = world[y][x];
	return tile == PROHIBITED_TILE || tile == PASSED_TILE;
}

bool valid(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	return x < width && y < height // x and y is unsigned
		   && !is_wall(world, x, y);
}

int walk(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	if (!valid(world, width, height, x, y)) {
		return 0; // ????????????????????\?????????????£?or??¢?´¢??????
	}
	int total = 1; // ????????????????????\?????????1?????¨??????????????????
	world[y][x] = PASSED_TILE; // ??????????????¨????????????????????¨?????????????????????????????????
	// ?????????????????§??????????????????valid???????????????walk????????§
	total += walk(world, width, height, x - 1, y);
	total += walk(world, width, height, x + 1, y);
	total += walk(world, width, height, x, y - 1);
	total += walk(world, width, height, x, y + 1);
	return total;
}

void process(size_t width, size_t height) {
	std::string world[MAX_HEIGHT];
	size_t x = std::numeric_limits<size_t>::max(), y = std::numeric_limits<size_t>::max();
	for (size_t i = 0; i < height; ++i) {
		std::cin >> world[i];
		size_t start_index = world[i].find(START_TILE);
		if (start_index != std::string::npos) {
			x = start_index;
			y = i;
		}
	}

	std::cout << walk(world, width, height, x, y) << std::endl;
}

int main() {
	size_t width, height;
	while (true) {
		std::cin >> width >> height;
		if (width == 0 || height == 0) {
			break;
		}
		else {
			process(width, height);
		}
	}
}

C++: #include <bits/stdc++.h>

using namespace std;

class P
{
public:
	int x,y;
	P(int X,int Y)
	{
		x=X;
		y=Y;
	}
};

int main()
{
	stack<P>st;
	int n,m;
	int numX[4]={0,1,0,-1};
	int numY[4]={1,0,-1,0};

	while(cin>>n>>m && n)
	{
		int flag[500][500]={};
		int count=0;
		string in;
		char set[500][500]={};

		for(int i=1;i<=m;i++)
		{
			for(int j=1;j<=n;j++)
			{
				cin >> set[i][j];
			}
		}

		for(int i=1;i<=m;i++)
		{
			for(int j=1;j<=n;j++)
			{
				if(set[i][j]=='@')
				{
					st.push(P(j,i));
					flag[i][j]=1;
				}
			}
		}

		int xx,yy;

		while(!st.empty())
		{
			count++;
			xx=st.top().x;
			yy=st.top().y;
			st.pop();
			for(int i=0;i<4;i++)
			{
				if(set[numY[i]+yy][numX[i]+xx]=='.' && flag[numY[i]+yy][numX[i]+xx]==0)
				{
					st.push(P(numX[i]+xx,numY[i]+yy));
					flag[numY[i]+yy][numX[i]+xx]=1;
				}
			}

		}
		cout << count << endl;
	}

    return 0;
}