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 <algorithm>
#define MAX_INT  100005
using namespace std;

//??\???????????§???
#define MAX_X  22
#define MAX_Y  22

//?????°
char field[MAX_X][MAX_Y];
int X, Y;
int px, py;                     // players position


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


void initialize_field() {
  for (int y = 0; y < Y; y++ ) {
    for (int x = 0; x < X; x++ ) {
      field[x][y] = '#';
    }
  }
}

void print_field() {
  for (int y = 0; y < Y; y++ ) {
    for (int x = 0; x < X; x++ ) {
      cout << field[x][y];
    }
    cout << endl;
  }
  cout << endl;
}


void dfs(int x, int y) {

  field[x][y] = '$';            // ?¨????????????¨?????°

  for (int i = 0; i < 4; i++){
    int nx = x + dx[i];
    int ny = y + dy[i];

    if(field[nx][ny] == '.') {
      dfs(nx, ny);
    }
  }

}


int main() {

  while(true) {

    initialize_field();

    cin >> X >> Y;

    if(X == 0 && Y == 0) break;

    for(int y = 0; y < Y; y++ ){
      for(int x = 0; x < X; x++ ){
        cin >> field[x][y];
        if( field[x][y] == '@' ) { px = x; py = y; }
      }
    }

    dfs(px, py);

    // ?¨?????????´????????°??????
    int res = 0;
    for(int y = 0; y < Y; y++ ){
      for(int x = 0; x < X; x++ ){
        if ( field[x][y] == '$' ) { res++; }
      }
    }
    cout << res << endl;
  }

  return 0;

}

C++: #include<iostream>

using namespace std;

char ban[21][21];
int cont = 0;

	int w, h;
	
	int MAX = 0;

void saiki(int y, int x)
{
	cont++;
	ban[y][x] = '_';
	/*
	for(int i = 0; i < h; i++){
		for(int j = 0; j < w; j++){
			cout << ban[i][j];
		}
		cout << endl;
	}
	
	cout << w << h << endl;
	
	getchar();
	
	cout << cont << endl;;
	*/
	
	if(ban[y+1][x] == '.'){
		saiki(y+1,x);
		ban[y+1][x] == '.';
	}
	if(ban[y-1][x] == '.'){
		saiki(y-1,x);
		ban[y-1][x] == '.';
	} 
	if(ban[y][x-1] == '.'){
		saiki(y,x-1);
		ban[y][x-1] == '.';
	}
	if(ban[y][x+1] == '.'){
		saiki(y,x+1);
		ban[y][x+1] == '.';
	}
	
	if(cont > MAX){
		MAX = cont;
	}
}

int main()
{
	for(;;){
		int st_x, st_y;
		MAX = cont = 0;
		
		cin >> w >> h;
		
		if( w == 0){
			break;
		}
		
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				cin >> ban[i][j];
				if(ban[i][j] == '@'){
					st_x = j;
					st_y = i;
				}
			}
		}
		
		
		saiki(st_y,st_x);
		
		cout << MAX << endl;
	}
}

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

using namespace std;

int xs[] = {0, 1, 0, -1}, ys[] = {1, 0, -1, 0};
char c[108][108];
int w, h;
bool come[108][108];
int dfs(int x, int y){
	if(x < 0 || y < 0 || x >= h || y >= w)return 0;
	if(c[x][y] == '#')return 0;
	if(come[x][y])return 0;
	come[x][y] = true;
	int res = 1;
	for(int i = 0;i < 4;i++){
		res += dfs(x + xs[i], y + ys[i]);
	}
	return res;
}

int main(){
	int si, sj;
	while(cin >> w >> h, w | h){
		for(int i = 0;i < h;i++){
			for(int j = 0;j < w;j++){
				cin >> c[i][j];
				if(c[i][j] == '@'){
					si = i;
					sj = j;
				}
				come[i][j] = false;
			}
		}
		cout << dfs(si, sj) << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <map>
using namespace std;
int main(int argc, char *argv[]) {
	int w,h;
	while (true) {
		cin>>w>>h;
		cin.ignore();
		if (w == 0 && h == 0) break;
		vector<string> map(h);
		for (int i=0;i<h;i++) getline(cin,map[i]);
		//for (int i=0;i<h;i++) cout<<map[i]<<endl;
		// search @
		int si,sj;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (map[i][j] == '@') {
					si = i; sj = j;
					break;
				}
			}
		}
		// bfs
		queue<pair<int,int> > mQue;
		int count = 0;
		mQue.push(make_pair(si,sj));
		vector<vector<bool> > check(h,vector<bool>(w,true));
		while (mQue.size() > 0) {
			int ti,tj;
			pair<int,int> tp = mQue.front();
			mQue.pop();
			ti = tp.first;
			tj = tp.second;
			//cout<<tj<<":"<<ti<<endl;
			// count
			if (check[ti][tj]) {
				count++;
				check[ti][tj] = false;
			}
			// deque
			if (ti-1>=0 && map[ti-1][tj] == '.' && check[ti-1][tj]) {
				mQue.push(make_pair(ti-1,tj));
			}
			if (tj+1<w && map[ti][tj+1] == '.' && check[ti][tj+1]) {
				mQue.push(make_pair(ti,tj+1));
			}
			if (ti+1<h && map[ti+1][tj] == '.' && check[ti+1][tj]) {
				mQue.push(make_pair(ti+1,tj));
			}
			if (tj-1>=0 && map[ti][tj-1] == '.' && check[ti][tj-1]) {
				mQue.push(make_pair(ti,tj-1));
			}
		}
		cout<<count<<endl;
	}
	return 0;
}