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 <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <cstring>

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

using namespace std;

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

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

        vector<string> s(h);
        rep(i,h) cin >> s[i];

        int sy,sx;
        rep(i,h) {
            rep(j,w) {
                if(s[i][j] == '@') {
                    sy = i;
                    sx = j;
                }
            }
        }
        
        int ans = 1;
        bool memo[25][25];
        memset(memo,0,sizeof(memo));

        queue<pair<int,int> > que;
        que.push(make_pair(sy,sx));
        memo[sy][sx] = true;
        
        while(que.size()) {
            pair<int,int> p = que.front();
            que.pop();
            
            rep(i,4) {
                int ny = p.first + dy[i];
                int nx = p.second + dx[i];

                if(0 <= ny && ny < h && 0 <= nx && nx < w) {
                    if(s[ny][nx] == '.' && !memo[ny][nx]) {
                        ans++;
                        memo[ny][nx] = true;
                        que.push(make_pair(ny,nx));
                    }
                }
            }
        }

        cout << ans << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <queue>
using namespace std;
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1};
int num =1,d[10000][10000];
char A[10000][10000];
bool valid(int x,int y,int W,int H)
{
    return (x>=0&&x<W)&&(y>=0&&y<H);
}
void Init(int W,int H)
{
    int i,j;
    num=1;
    for(i=0;i<W;i++)
        for(j=0;j<H;j++)
            d[i][j]=0;
}
void dfs(int x, int y, int W, int H, char A[10000][10000])
{
    int i;
    for(i=0;i<4;i++)
    {
        if(valid(x+dx[i],y+dy[i],W,H))
        {
            if(A[x+dx[i]][y+dy[i]]=='.' && d[x+dx[i]][y+dy[i]]==0)
            {
                num++;
                A[x+dx[i]][y+dy[i]]='#';
                d[x+dx[i]][y+dy[i]]=1;
                dfs(x+dx[i],y+dy[i],W, H,A);
            }
        }
    }
}
int main()
{
    int W,H,i,j,x,y;
    queue<int> Q;
    cin >> H >> W;
    while(W && H)
    {
        for(i=0;i<W;i++)
        {
            for(j=0;j<H;j++)
                cin >> A[i][j];
        }
        for(i=0;i<W;i++)
            for(j=0;j<H;j++)
            {
                if(A[i][j]=='@')
                {
                    x=i;
                    y=j;
                }
            }
        Init(W,H);
        d[x][y]=1;
        dfs(x, y, W, H, A);
        cout << num << endl;
        cin >> H >> W;
    }
    return 0;
}