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 <stdio.h>
#include <algorithm>
#include <vector>
#include <queue>
#include <numeric>
#include <string>
#include <string.h>
#include <map>
#include <set>
#include <functional>
#include <complex>
#include <stack>
#include <tuple>
#include <array>
using namespace std;

#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define ALL(u) begin(u),end(u)
#define PB push_back
#define LE(n,m) ((n) < (m) + EPS)
#define GE(n,m) ((n) + EPS > (m))
#define EQ(n,m) (abs((n)-(m)) < EPS)

typedef long long int ll;
const int INF = (1<<30) - 1;
const double EPS = 1e-9;
const int MOD = 1000000007;

int W, H;
int dw[] = {0, 0, 1, -1};
int dh[] = {1, -1, 0, 0};

int dfs(vector<string> &field, int h, int w){
    field[h][w] = '#';
    
    int res = 1;
    for(int i=0; i<4; i++){
        int nh = h + dh[i];
        int nw = w + dw[i];
        if(0<=nh && nh<H && 0<=nw && nw<W && field[nh][nw]=='.'){
            res += dfs(field, nh, nw);
        }
    }
    return res;
}

//g++ -std=c++0x -msse4.2 -O3
//#include <bits/stdc++.h>
int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    //cout.precision(16);
    //cout.setf(ios::fixed);
    
    while(cin >> W >> H, W|H){
        vector<string> field(H);
        for(auto &l: field)
            cin >> l;
        
        for(int i=0; i<H; i++)
            for(int j=0; j<W; j++) if(field[i][j] == '@')
                cout << dfs(field, i, j) << endl;
    }
    
    
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;char n[100][100];int k[100][100],l[100][100],sum,H,W,i,j,m,s;
#define A(B,C) if(k[B][C]!=1){l[B][C]=1;}
main(){while(true){s=0;memset(l,0,sizeof(l));cin>>W>>H;if(W==0){break;}for(i=1;i<=H;i++){for(j=1;j<=W;j++){cin>>n[i][j];if(n[i][j]=='.'){k[i][j]=0;}else if(n[i][j]=='@'){k[i][j]=2;l[i][j]=1;}else{k[i][j]=1;}}}for(i=0;i<2500;i++){for(j=1;j<=H;j++){for(m=1;m<=W;m++){if(l[j][m]==1){A(j-1,m);A(j+1,m);A(j,m-1);A(j,m+1);}}}}for(j=1;j<=H;j++){for(m=1;m<=W;m++){if(l[j][m]==1){s++;}}}cout<<s<<endl;}}

C++: #include <iostream>
using namespace std;

int moved = 0;
int w, h;
char myMap[20][20];

bool safe(int x, int y){
    return x < h && y < w;
}

void move(int x, int y){
    myMap[x][y] = '#';
    moved++;
    if (myMap[x+1][y] == '.' && safe(x+1, y)) move(x+1, y);
    if (myMap[x-1][y] == '.' && safe(x-1, y)) move(x-1, y);
    if (myMap[x][y+1] == '.' && safe(x, y+1)) move(x, y+1);
    if (myMap[x][y-1] == '.' && safe(x, y-1)) move(x, y-1);
    return;
}

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

	moved = 0;
	int initX, initY;
	
	for (int i = 0; i < h; i++){
	    for (int j = 0; j < w; j++){
		cin >> myMap[i][j];
		if (myMap[i][j] == '@'){
		    initX = i; initY = j;
		}
	    }
	}
	
	move(initX, initY);
	
	cout << moved << endl;
    }
}

C++: #include <iostream>
#include <cstdio>
#include <string>
#include <queue>
using namespace std;

typedef pair<int, int> P;

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

int sx, sy;
int ans;
int w, h;
vector<string> ma;

void BFS(){
	queue<P> que;
	que.push(P(sy, sx));
	ans++;

	while(que.size()){
		P p = que.front(); que.pop();

		for(int i=0; i<4; i++){
			int ny = p.first + dy[i];
			int nx = p.second + dx[i];

			if(ny >= 0 && ny < h && nx >= 0 && nx < w && ma[ny][nx] == '.'){
				ans++;
				ma[ny][nx] = '#';
				que.push(P(ny, nx));
			}
		}

	}

}

int main(void){
	while(1){
		ma.clear();
		ans = 0;
		cin >> w >> h;
		if(w == 0 && h == 0) break;

		for(int i=0; i<h; i++){
			string tmp;
			cin >> tmp;

			ma.push_back(tmp);
		}

		for(int i=0; i<h; i++){
			for(int j=0; j<w; j++){
				if(ma[i][j] == '@' ){
					sy = i;
					sx = j;
				}

			}
		}

		BFS();

		cout << ans << endl;


	}


	return 0;
}

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

#define REP(i, n) for(int i = 0;i < (n);i++)
#define FOR(i, a, b) for(int i = (a);i < (b);i++)
#define all(v) v.begin(),v.end()

using int64 = long long;

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

int main(void){
	int32 H, W;
	while(cin >> H >> W && H && W){
		vector<string> f(H+2, string(W+2, '#'));
		int sy, sx;
		REP(i, H){
			cin >> f[i+1];
			REP(j, W+2)
				if(f[i+1][j] == '@'){
					sy = i+1;
					sx = j;
				}
			f[i+1][0] = f[i+1][W+1] = '#';
		}
		bool visited[30][30] = {};
		int cnt = 0;
		
		auto dfs = [&](int y, int x){
			visited[y][x] = 1;
			cnt++;
			REP(i, 4){
				int yy = y+dy[i], xx = x+dx[i];
				if(f[yy][xx] != '#' && !visited[yy][xx]){
					dfs(yy, xx);
				}
			}
		};
		dfs(sy, sx);
		cout << cnt << endl;
	}
}