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>
using namespace std;

struct Point
{
  int i;
  int j;
};

static Point minus_point = { -1, -1 };

class Map
{
  public:
    Map(int height, int width);
    ~Map();

  public:
    int Can_go_num(Point p = minus_point);

  private:
    bool CheckIndex(const Point& p);
    bool isBlack(const Point& p);
    bool CanMove(const Point& p);

  private:
    char** m_map;   //  '.' = black, '#' = red, '@' = start, 'x' = already
    int m_height;
    int m_width;
    Point m_start;
};

Map::Map(int height, int width)
{
  m_height = height;
  m_width = width;

  m_map = new char*[m_height];
  for (int i = 0; i < m_height; ++i)
  {
    m_map[i] = new char[m_width];
  }

  for (int i = 0; i < m_height; ++i)
  {
    for (int j = 0; j < m_width; ++j)
    {
      cin >> m_map[i][j];
      if (m_map[i][j] == '@')
      {
        m_start.i = i;
        m_start.j = j;
      }
    }
  }
}

Map::~Map()
{
  for (int i = 0; i < m_height; ++i)
  {
    delete[] m_map[i];
  }
  delete[] m_map;
}

int Map::Can_go_num(Point p)
{
  int result = 1;
  if (p.i == -1)
  {
    p = m_start;
  }
  m_map[p.i][p.j] = 'x';
  Point buf_ps[4] = {
    { p.i, p.j-1 },
    { p.i, p.j+1 },
    { p.i-1, p.j },
    { p.i+1, p.j },
  };
  for (Point buf_p : buf_ps)
  {
    if (CanMove(buf_p) )
    {
      result += Can_go_num(buf_p);
    }
  }

  return result;
}

inline bool Map::CheckIndex(const Point& p)
{
  return (0 <= p.i and p.i < m_height) and (0 <= p.j and p.j < m_width);
}

inline bool Map::isBlack(const Point& p)
{
  return m_map[p.i][p.j] == '.';
}

inline bool Map::CanMove(const Point& p)
{
  return CheckIndex(p) && isBlack(p);
}

int main(void)
{
  int height, width;
  while (true)
  {
    cin >> width >> height;
    if (height == 0 && width == 0) break;
    Map map(height, width);
    cout << map.Can_go_num() << endl;
  }
  return 0;
}

C++: #include<iostream>
#include<string>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
using namespace std;
typedef long long ll;
const ll MOD = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
typedef unsigned int ui;
int main(){
	int w, h;
	string s;
	int i, j;
	while (cin >> w >> h) {
		if (w == 0)break;
		char tile[22][22]; int used[22][22];
		for (i = 0; i < 22; i++) {
			tile[i][0] = '#';
			tile[i][h + 1] = '#';
		}
		for (j = 0; j < 22; j++) {
			tile[0][j] = '#';
			tile[w + 1][j] = '#';
		}
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				used[i][j] = 0;
			}
		}
		vector<int> q1,q2;
		for (j = 1; j <= h; j++) {
			cin >> s;
			for (i = 1; i <= w; i++) {
				tile[i][j] = s[i - 1];
				if (s[i - 1] == '@') {
					q1.push_back(i); q2.push_back(j);
				}
			}
		}
		while (!q1.empty()) {
			int x = q1[0]; int y = q2[0]; used[x][y]=1;
			q1.erase(q1.begin() + 0); q2.erase(q2.begin() + 0);
			if (used[x - 1][y] == 0 && tile[x - 1][y]=='.') {
				q1.push_back(x - 1); q2.push_back(y);
			}
			if (used[x + 1][y] == 0 && tile[x + 1][y] == '.') {
				q1.push_back(x + 1); q2.push_back(y);
			}
			if (used[x][y-1] == 0 && tile[x][y-1] == '.') {
				q1.push_back(x); q2.push_back(y-1);
			}
			if (used[x][y+1] == 0 && tile[x][y+1] == '.') {
				q1.push_back(x); q2.push_back(y+1);
			}
		}
		int count = 0;
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				if (used[i][j] == 1)count++;
			}
		}
		cout << count << endl;
	}
	return 0;
}

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

vector<string> f;
void init(){
  f.clear();
}

int input(){
  int w,h;
  cin>>w>>h;
  if(w==0&&h==0)return false;
  string s;
  for(int i=0;i<h;i++){
    cin>>s;
    f.push_back(s);
  }
  return true;
}

int vx[] = {0,0,1,-1};
int vy[] = {1,-1,0,0};

int rec(int x,int y){
  if(x<0||f.size()<=x)return 0;
  if(y<0||f[x].size()<=y)return 0;
  if(f[x][y]!='.')return 0;
  f[x][y]='#';
  int res = 1;
  for(int i=0;i<4;i++){
    res += rec(x+vx[i],y+vy[i]);
  }
  return res;
}

int solve(){
  int x,y;
  for(int i=0;i<f.size();i++){
    for(int j=0;j<f[i].size();j++){
      if(f[i][j]=='@'){
        x = i;
        y = j;
        f[i][j] = '.';
      }
    }
  }
  return rec(x,y);
}

int main(){
  while(init(),input()){
    cout<<solve()<<endl;
  }
}

C++: #include <bits/stdc++.h>
using namespace std;
char maze[21][21];
#define r(z,x) for(int z=0;z<x;z++)
int ans;
void dfs(int x,int y){
	int dx[4]={0,-1,0,1};
	int dy[4]={1,0,-1,0};
	maze[x][y]='#';
	ans++;
	r(i,4)if(maze[x+dx[i]][y+dy[i]]=='.')dfs(x+dx[i],y+dy[i]);
}
int main(){
	int a,b;
	while(cin>>a>>b,a||b){
		ans=0;
		fill(maze[0],maze[20],'#');
		r(i,b)r(j,a)cin>>maze[i][j];
		r(i,b)r(j,a)if(maze[i][j]=='@')dfs(i,j),cout<<ans<<endl;
	}
}