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<cstdio>
#include<array>
#include<vector>

using namespace std;

vector<vector<int>>tile(22, vector<int>(22));

array<int, 4>mx = { -1,0,0,1 };
array<int, 4>my = { 0,-1,1,0 };

void walk(int x, int y) {
	tile[x][y] = 1;
	for (int i = 0; i < 4; i++) {
		if (tile[x + mx[i]][y + my[i]] != 0)continue;
		walk(x + mx[i], y + my[i]);
	}
}

int main() {
	int w, h;
	while (cin >> w >> h, w) {
		int stx, sty;
		for (auto &i : tile) {
			for (auto &j : i) {
				j = -1;
			}
		}
		for (int i = 1; i <= h; i++) {
			for (int j = 1; j <= w; j++) {
				char ti;
				cin >> ti;
				if (ti == '.')tile[j][i] = 0;
				else if (ti == '#')tile[j][i] = -1;
				else {
					tile[j][i] = 1;
					stx = j; sty = i;
				}
			}
		}
		walk(stx, sty);
		int ans = 0;
		for (auto i : tile) {
			for (auto j : i) {
				if (j == 1)ans++;
			}
		}
		printf("%d\n", ans);
	}
}

C++: #include <iostream>
#define rep(i,a,b) for(int i=(a); i<(b); i++)

using namespace std;

int w,h;
char m[22][22];
int dx[] = {1,-1,0,0};
int dy[] = {0,0,1,-1};

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

int main(){
  while(cin >> w >> h , w + h){
    int sx,sy;
    rep(i,0,h){
      rep(j,0,w){
        cin >> m[i][j];
        if(m[i][j] == '@'){
          sx = j;
          sy = i;
        }
      }
    }
    cout << dfs(sy , sx) << endl;
  }
  return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <functional>
using namespace std;
#define MOD 1000000007
#define ADD(X,Y) ((X) = ((X) + (Y)%MOD) % MOD)
typedef long long i64; typedef vector<int> ivec; typedef vector<string> svec; typedef pair<int, int> pi;
int dd[] = { 0, 1, 0, -1 };


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

		char C[30][30];
		int B[30][30];
		for (int i = 0; i < H; i++)
		{
			scanf("%s", C[i]);
		}
		int sx = -1; int sy = -1;
		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				B[i][j] = 0;
				if (C[i][j] == '@')
				{
					sy = i;
					sx = j;
				}
			}
		}

		queue<pi> Q;
		Q.push(pi(sy, sx));
		B[sy][sx] = 1;
		int ans = 1;
		while (!Q.empty())
		{
			int ty = Q.front().first; int tx = Q.front().second;
			Q.pop();

			for (int r = 0; r < 4; r++)
			{
				int y = ty + dd[r];
				int x = tx + dd[r + 1];
				if (y < 0 || y >= H || x < 0 || x >= W)continue;
				if (C[y][x] == '#')continue;
				if (B[y][x])continue;
				ans++;
				B[y][x] = 1;
				Q.push(pi(y, x));
			}
		}

		cout << ans << endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int (i)=(0);(i)<(int)(n);++(i))
using ll = long long;
using P = pair<int, int>;
using namespace std;

template<class T> void vin(vector<T>& v, int n) {
    v.resize(n);
    for (int i = 0; i < n; ++i) {
        cin >> v[i];
    }
}

int w, h;
char ch[50][50];
int dx[] = { 0, 1, 0, -1 };
int dy[] = { 1, 0, -1, 0 };

void bfs(int y, int x) {
    rep(i, 4) {
        int ny = y + dy[i], nx = x + dx[i];
        if (0 > ny or 0 > nx or ny >= h or nx >= w) continue;
        if (ch[ny][nx] == '.') {
            ch[ny][nx] = 'a';
        }
    }
}

int main() {
    while (cin >> w >> h and w and h) {
        int py = 0, px = 0;
        rep(i, h) rep(j, w) {
            cin >> ch[i][j];
            if (ch[i][j] == '@') {
                py = i, px = j;
                ch[i][j] = '.';
            }
        }
        bfs(py, px);
        int ans = 0;
        rep(i, h) rep(j, w) if (ch[i][j] == 'a') ans++;
        cout << ans << endl;
    }
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef vector<string> V;
typedef pair<int, int> P;
typedef queue<P> Q;
int main() {
	int w, h, x, y;
	cin >> w >> h;
	if (w == 0 && h == 0) {
		return 0;
	}
	V map = V(h);
	for (int i = 0; i < h; i++) {
		cin >> map[i];
		for (int j = 0; j < w; j++) {
			if (map[i][j] == '@') {
				x = j;
				y = i;
			}
		}
	}
	int n = 0;
	Q q;
	q.push(P(y, x));
	while (q.size()) {
		P p = q.front();
		q.pop();
		if (p.first < 0 || h <= p.first || p.second < 0 || w <= p.second || map[p.first][p.second] == '#') {
			continue;
		}
		//cout << p.second << " " << p.first << " " << map[p.first][p.second] <<endl;
		map[p.first][p.second] = '#';
		n++;
		q.push(P(p.first - 1, p.second));
		q.push(P(p.first + 1, p.second));
		q.push(P(p.first, p.second - 1));
		q.push(P(p.first, p.second + 1));
	}
	cout << n << endl;
	return main();
}