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 <vector>
#include <fstream>
#include <algorithm>
#include <iomanip>
#include <map>
#include <queue>
#include <stack>
#include <set>
#include <ctime>
#include <iterator>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
using namespace std;

int main()
{
    int nb_lignes,nb_colonnes;
    while(cin>>nb_lignes>>nb_colonnes)
    {
        if(nb_lignes == 0)
            break;
        int res = 1;
        vector<string> lab(nb_lignes);
        vector<vector<bool> > visites(nb_lignes,vector<bool>(nb_colonnes,false));
        for(int c=0;c<nb_lignes;c++) cin>>lab[c];
        int xdep,ydep=0;
        for(xdep=0;xdep<nb_lignes&&lab[xdep][ydep]!='@';xdep++){ for(ydep=0;ydep<nb_colonnes&&lab[xdep][ydep]!='@';ydep++);if(lab[xdep][ydep]=='@')break;}
        vector<pair<int,int> > parcours;
        visites[xdep][ydep]=true;
        parcours.push_back(make_pair(xdep,ydep));
        while(parcours.size())
        {
            int xact = parcours.back().first;
            int yact = parcours.back().second;
            parcours.pop_back();
            int dx[4]={1,-1,0,0};
            int dy[4]={0,0,1,-1};
            for(int c=0;c<4;c++)
            {
                int x2 = xact + dx[c];
                int y2 = yact + dy[c];
                if(x2>=0&&x2<nb_lignes&&y2>=0&&y2<nb_colonnes&&lab[x2][y2]!='#'&&!visites[x2][y2])
                {
                    visites[x2][y2]=true;
                    res++;
                    parcours.push_back(make_pair(x2,y2));
                }
            }
        }
        cout<<res<<endl;
    }
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <queue>

using namespace std;
using Tiles = vector<string>;

struct Point {
	int x;
	int y;

	Point() :
		x{0}, y{0}
	{}

	Point(int _x, int _y) :
		x{_x}, y{_y}
	{}

	const Point operator+ (const Point& rhs) const
	{
		Point temp{x + rhs.x, y + rhs.y};
		return temp;
	}
};

const Point Arround[4] {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};

int dfs(const Tiles& tiles, const Point& start, int W, int H)
{
	int count{0};
	queue<Point> Q;

	// ????????¨????????????????????????????????????
	vector<vector<bool>> done(H, vector<bool>(W, false));

	Q.push(start);
	done[start.y][start.x] = true;

	while(true) {
		if(Q.empty()) break;

		const auto& pos = Q.front();
		Q.pop();

		if(tiles[pos.y][pos.x] == '#') continue;

		++count;

		for(const auto& a : Arround) {
			const auto& p = pos + a;

			if(p.x < 0 || W <= p.x) continue;
			if(p.y < 0 || H <= p.y) continue;
			if(done[p.y][p.x]) continue;

			done[p.y][p.x] = true;
			Q.push(p);
		}
	}

	return count;
}

int main()
{
	int W, H;

	while(true) {
		cin >> W >> H;
		if(W == 0 && H == 0) break;

		Tiles tiles(H);
		Point start;

		for(int i = 0; i < H; ++i) {
			cin >> tiles[i];

			for(int j = 0; j < tiles[i].size(); ++j) {
				if(tiles[i][j] == '@') {
					start = Point{j, i};
					break;
				}
			}
		}

		cout << dfs(tiles, start, W, H) << endl;
	}

	return 0;
}

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

#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 rer(i, a, b) for (int i = (int)a; i <= (int)b; ++i)
#define each(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define all(v) v.begin(), v.end()
#define mset(a, n) memset(a, n, sizeof(a))

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vector<int> > vvi;
typedef vector<pair<int, int> > vpii;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int inf = 1000000000;
const int mod = 1000000007;
const double eps = 1e-9;
const int dx[] = { -1, 0, 1, 0};
const int dy[] = { 0, -1, 0, 1};

string field[100];

void dfs(int y, int x) {
	if (x < 0 || y < 0 || x == w || y == h) return;
	if (field[y][x] == '#') return;
	ans++;
	field[y][x] = '#';
	rep(i, 4) dfs(y + dy[i], x + dx[i]);
}

int main() {
	int W, H;
	while (cin >> W >> H, W) {
		int ans = 0;
		rep(i, H) cin >> s[i];
		rep(i, H) rep(i, W) {
			if (s[i][j] == '@') solve(i, j);
		}
	}
	return 0;
}